I'm getting old and far-sighted and focusing is a nightmare. I often forget to bring the right strength of glasses with me and they're always in the way when I do remember to bring them along. Being a Bronica owner, coming across diopters that suits my eyesight isn't easy.
So, here's a receipe on how to make your own custom diopters in 30 minutes for the price of a pair of cheap gas-station reading glasses.
This method should suit well for all kinds of cameras and their prism-finders and making diopters for waist level finders shouldn't be that hard either.
To find the correct strength of glasses, choose the strength of glasses that makes you see through the prism-finder sharply.

A pair of cheap reading glasses and calipers and a needle.
Measure the original diopter's opening with the calipers and place it on the glasses.
Scribe two paralell lines.

Turn the calipers 90 degrees and repeat the process.

Here's what you should end up with:

Cut out the square using a fine-toothed modellers saw, file the sides if they're uneven and press it in place on top of the original diopter and that's it.
No more focusing trouble for us older guys and girls.

Making diopters for WLF's is only a matter of finding glasses large enough to cover the original enlarging lens. A tiny drop of epoxy-glue will hold it in place.
Don't use super-glue. The vapours will make a frosted diopter. Not good . . .