I made a fine 16x loupe from the rear element of a crappy zoom lens that pooped out on me several years ago.

Mounted the element, using its two threaded mount rings, into the top of a plastic box my favorite local lab uses for 35mm slides. The element-to-film distance was darned near perfect. All I hafta do to focus precisely if vary the finger pressure I put on the lid.

The rest of that nasty lens I scrapped.