How to secure your FOB?

Is there an optimal strategy to secure your FOB against raids if you are off line (not playing the game) most of the time?

I have a very limited time to play games and on line raids appeal to me about as much as stabbing myself in the face with a rusty spoon. While it might be possible to finished the game without using any FOB, it is not clear that you can avoid building one.

If it turns out that I have to build one and open myself to abuse from other players: What is best way I can set up my base so that even if a raid is successful, I do not lose anything important? I will probably not be on line to defend my base and thus have to rely on whatever defences I can set up.

The important thing if you don't want to be invaded is taking the incentive away from the potential invader, by limiting their potential gains and by increasing security to make it harder for them to get to the goal. Potential attackers will likely pick a different, easier target with greater spoils instead of yours.

When you initially build it, it will just build the command platform. The rewards for invading a department with a single platform is very low, the invader wont get any volunteers, and the GMP reward is usually lower than the cost of their deployment. Also (with the exception of the base development platform) there will be no containers to steal on single platform department so I'd say it's unlikely people will invade at all. There's very little incentive for invading small FOBs.

The base development platform is where the majority of your online unprocessed materials are kept, so it is a common target for attackers. If you don't create the platform though, then people can't steal from it.

To further put people off, I'd recommend building security items like drones and mines, upgrading equipment to 5* that the security team can use (weapons and items marked in green), and putting the max number of people on the platform from the security menu on your iDroid.

Capping the skill level of the defence team in the security section will take away the incentive of an invader going in to steal the guards and it will mean you have less to lose if they decide to kill/extract a lot of staff. (If you set it to B for example the highest grade soldiers it will use from your security team will be B, and if a few die/get extracted it's not a big deal. You can set this at whatever level you're happy with).

I think if you just keep a level 1 command platform, maybe level 1 r&d and combat platforms, I don't think you'd get any invaders- and you don't really have much at risk even if they do.

Really I wouldn't worry, the reward for defences is really high. If they successfully invade you might lose a few staff, but successful defence I've gotten up to 700,000GMP, multiple S ranked volunteers, processed materials and plants. So I think you will likely receive a net gain for participating in FOBs. I've often won FOB defences when I was offline just because the invader got spotted and got nailed by the guards.

Also from the relationship menu try adding as many people from the random section as you can as some of them will hopefully add you back. people that support you have the option to defend your FOB and as the reward for a successful defence is so high, and defending is so easy- people likely will do that (and you'll have the option to defend theirs for a reward as well if they get attacked).

Another point is if you don't want to be invaded, don't invade other people. If you are detected invading it will inform the other person and give them an option to attack you back for a limited time (all their supporters have the option to attack you too), and usually they will.

If you want to opt out completely (including daily bonuses etc.) I've heard there are some TOS you can decline which will take you completely offline, but I've not tried that myself.

TL;DR: Don't worry, unless you build up your fob you don't have much at risk

I can tell you what is the most effective strategy (at least at low levels) from invading a few times myself.

1. Fill your security staff with all of your worst Combat soldiers (Ds and Es). This will ensure none of your good combatants are stolen, and it really doesn't seem to make that much of a difference for guard duty.
2. If you have extra GMP, install decoys. Decoys can cause an invader to screw up and attract guards if they're not paying close enough attention, but the "real" (more reliable) benefit is that the enemy can't know your true guard count because adding decoys inflates the number of guards listed on your platform.
3. Once you inevitably develop it to the point that it's actually worth defending, security devices aren't exorbitantly expensive and make the invader's job much harder. It's easy to pay attention to guards and decoys. It's hard to pay attention to guards, decoys, IR gates, drones, mines... the margin for error becomes much smaller.
4. Hold off building your Base Development Platform. When people invade it, it's basically just a giant warehouse filled with all of your resource containers, so you give them the opportunity to steal them from you.
5. If you do have to build a new platform, stack decks on it. Decks make your security exponentially tighter; when someone invades your platform, they start on the outermost deck. In other words, if you have 4 decks, an invader will always have to go through 4 entire decks to get to the goal point. Not to mention, your guard capacity is limited by your decks, so you have roughly quadruple the guards of a guy with 1 deck.

And, GP89 is correct when he says that it's incredibly easy for someone to make a mistake and give you an instant win, even on minimum security platforms... my last invasion ended in a suicide after I accidentally pressed the stance button while climbing on a pipe. I was spotted and panicked; I hit the stance button because I thought I would slide down like on ladders, but I fell five stories onto the deck and cratered. Easy money for the defender.

Invasions are tough relative to normal missions. There's just so many angles that guards can spot you from with all the catwalks, and pipes are the absolute worst climbing device -- they're like slow ladders that you can't safely exit quickly and leave you completely helpless. You have the advantage as a defender. Also, keep in mind that in conventional missions, you can escape much more easily from sticky situations due to the open nature of the terrain. Decks are constrained to quite limited areas, with some underhanging catwalks to hide in. Hiding places are few and far between, so you're virtually always either dodging a guard or taking one out, or both. This game isn't quite like Dark Souls so you don't need to worry as much if you get invaded.

P.S.: The invader also loses heroism instantly upon killing your soldiers. I tried the "panther" approach (just take silenced lethal weapons, sneak as fast as possible and shoot anyone that spots you) and I stopped as soon as I saw that happen.