Summary

I have this old Industar 50/2 (f:3.5/50mm) camera lens.
I searched a lot, but found no 35.9mm lens cap to sell. So I modeled it on Blender and printed it.
I also printed a back lens cap for it. A standard M42 screw mount that can work on other lenses as well.
These caps I recommend the printing in TPU due to it's flexibility.