A term for a particular makeshift lock (image included)

It's a Cabin Hook.

cabin hook noun

Definition of CABIN HOOK

: a small hook and eye for use on cabinet doors

First Known Use of CABIN HOOK 1845

Source http://www.merriam-webster.com/dictionary/cabin%20hook

They are also used to hold outward opening doors (for example patio doors) against a wall (to stop the doors blowing closed in a strong wind).

Also used to fasten outward opening doors on boats.

Some examples:

I think the word you want is latch.

http://dictionary.reference.com/browse/latch

Lock would imply you need a key or some other device to open it.

This is not a makeshift lock!

1. Makeshift does not mean 'simple', 'basic' or 'primitive'! It means 'created in an emergency out of whatever materials are to hand'. For example, you might take off your white shirt and tie it to a pole to create a makeshift flag of surrender.

2. A 'lock' is a secure closing device operated by a key or a numerical combination. What you have in the picture is a latch; specifically a 'hook-and-eye' latch.

This is called either a hook and eye lock or a cabin door lock. Your picture looks like the hardware pivots into the room, in which case it's called a cabin swivel door lock.