A simple door lock to use from inside to prevent door opening, even with a key:

With this kind of doors locks, the two steel bolts shown on the first picture move towards each other. when the door is opened. With the printed device, this movement is effectively prevented, the bolts are kept in place. Therfore, the door can not be opened, even with the right key. It opens a few milimeters, but this is it. The bolt still locks the door.
Of course, the device can only lock the door from inside. Install, when the door is locked.
Please note, that you should secure the cabin doors and windows as well. For the windows, see my other design.

23.10.20: Updated design for easier printing and installation.