Plastic pieces to put in the metal holes in the door frame.
The piece is tapered, such that it expands when a bolt is screwed in. It's a bit loose when there's no bolt screwed in, but when a bolt is screwed in it sits damn tight.
OpenSCAD was used to create this, use that if you want to modify the bolt diametre and such.
This probably fits other purposes as well.
The author marked this model as their own original creation.