Originally Posted by wabashabi
The gamma lid oring is actually just o-ring cord spliced I believe. At least mine are. If you look close you can see where the ends are glued. You can buy oring cord sold by the foot of various types. Guessing Buna-N might be the best option. Silicone would take a higher temperature but not sure it would hold up as well. I've never tried making my own orings, so cannot claim they will take the abuse. This website was the first to come up on google, but might be a cheaper option out there.
Chemical Resistance Guide : The O-Ring Store LLC, We make getting O-Rings easy!
Yup, it can be done. We used to make custom O-rings where I used to work. From roughly the size of a gamma seal to about 3' dia (about 1/2" thick). Not hollow though, they were solid and I think if I were to make a gamma seal O-ring, I'd rather make it solid, anyway.
Here's a short video that shows how to do it with a jig you can buy (from the website above), but you can also do it the way I describe below.
Anyway, all you do, is take a NEW single edge razor blade, put the O-ring cord on a scrap of wood, then while holding the O-ring firmly, push down with the razor blade (kinda like slicing a block of cheese), trying to make the squarest cut you can. Practice it a few times, til you're getting a good square cut, then take your measurement and make the final cut. Then glue it together with a drop of superglue. I like to use a scrap of wax paper (to keep it from sticking), then hold it firmly til it's set. A little v-block helps center the joint, but it's not necessary. Then gently sand the glue bead (or slight misalignment) on the joint with 200-300 grit paper til it's pretty smooth. Though the joint doesn't often break, they occasionally do, so keep superglue, sandpaper and razor blades in your tool kit.