boojum
Mentor
I just ran across this for 3D printing a lens. A lens that apparently works. Odd focal length but if you have the 3D printer it looks like $15 will get you in the game: https://petapixel.com/2022/08/22/you-can-3d-print-and-build-this-164mm-f-2-5-lens-for-less-than-15/