Imageframe glasholder 18x24cm with included spring

This a simple solution for picture-frame glasplates. I don't want to cut the glasplates from the
buyable image frames (24x18cm 2mm is a standard size in Germany - costs around 1-2EUR/piece) and so
screwable solutions don't work good and bulldogclamps need too much place. Here are 4 corners
(marked with FR, FL, RR and RL - FrontRight, FrontLeft, RearRight and RearLeft),
attached to the corner screws under the bed (you need additional 4 x M3 nuts and fitting washers).
The glasplate will