First of all, I needed something to secure all the hardware to the lock. I figured picking the thing already was shady enough and I didn't want to make it worse for my friend by drilling in or otherwise damaging the door. I eventually settled on two aluminium L-shaped profiles, which when connected together would cleanly fit over the locks faceplate. I now had a nice base to build up from.
After some thinking and waaaay too much fiddling with gear trains to build up the torque to shift the lock with a simple electromotor, I decided my mechanical skills just suck, and I should try something simpler. I figured grabbing a larger motor should work just fine, and I could then use something as simple as a rubber band to connect the force to the door knob. With that in mind I went through my part-bins to look for a motor and a rubber band, but I couldn't find any. No problem, we've got a store that sells miscellaneous second-hand stuff a few minutes away, and they happened to have some old inkjet printers. I took one for the hefty sum of three euros, and it happened to contain just what I needed: a sturdy stepper motor and a dented rubber belt to go with it. I screwed the motor to the frame, and while the rubber band now had to make a 90 degrees angle, the whole contraption now was a lot simpeler.
One thing left: the rubber band was waaaay too long, so it should be made into
a shorter one. If the belt would move something continuously, this would be a
problem: connecting the two loose ends together again while keeping the dent space
intact would be a huge pain. Luckily, for our application the belt had to move
ust enough to open and close the lock, so I could be a bit more crude: