That's a great idea ...
I manage to spot quite identical orthodontic colored and non-colored elastic latex bands on eBay 3/16" (4.8mm). I was going to order the 1/4" but I ended up ordering the 3/16" (4.8mm) to stretch them enough so the thickness of the ring at the end would be between 0.3mm - 1 mm, I'd like ~0.5mm but I don't have the o-ring in front of me yet.
After ordering the orthodontic rings, I spotted on Amazon.com "Light Baffle" that is 1/32" (0.8mm). If you ever look inside a camera with a mirror, this material is the thin layer that protects the light to go behind the mirror. I'm posting the link below for those you might want to "craft" the o-rings :
Amazon : search for => Light Baffle, .03125" - 1/32" <= there's 1 model with adhesive on single side , sold by Micro-Tools.
Micro-Tools USA (part # LB132) :
http://www.micro-tools.com/store/P-LB132/Light-Baffle-03125--132.aspxMicro-Tools EU :
http://www.micro-tools.de/ , search for "light baffle".
Any repair camera center should be able to provide you that material. Note : 1/32" => 0.79mm
I do have a concern with the elastic band, will they dry overtime and break? I know standard elastic do dries and then break easy. The orthodontic "o-ring" uses medical latex. There are many ways to do latex and all end product have different specifications. I might end up ordering the "camera mirror light baffle". In USA on Amazon the size is 10" x 10" x 1/32" -- more than enough for 60 keys.
PBR