How can I force my iPad to lock?
Solution 1:
There is unfortunately no way to do this on a situational basis. My suggestion would be to just set it to immediately and get used to entering in your passcode every time. It'll be a habit before you know it!
Solution 2:
Turning the iOS device off will require a passcode to be entered no matter the timeout value if you have one other than requiring the passcode immediately upon locking.
You can also utility any of the "Find my iPhone" or MDM tools that allow remote locking of the device, but these are often not feasible and much more time and work than simply powering off the device or rebooting it.