Well I see a lot of views, but no answers so I will share what I have discovered.
I was trying to think of a way to make my own and I discovered something that almost all of us must have had at one point or another - the good old cartomizer condom! This seemed to be perfect since it was already about the same thickness as the original o-ring and if I could cut a sliver off, it might just work.
I first tried cutting one by snipping a sliver off the end with scissors - not too uniform in thickness, but it works! I have been testing one for a couple of days now and there are no leaks whatsoever. During testing I blew hard into the tip, sucked (watch it now!) on the tip and kept it fully filled and have experienced no leaks.
The condom is only slightly smaller in diameter than the original o-ring, but stretches to fit. This causes the thickness of it to diminish somewhat, but so what - it doesn't leak.
Since then I devised a technique to make more uniformly thick rings. I'll give you a quick overview and maybe you can come up with your own way to cut it. I figured that if I could somehow put a cylindrical metal piece around the condom, I could use the edge of the metal as a straight-edge to cut against. I used to reshaft golf clubs and the stepped shaft seemed to be a perfect tool. I cut one of the segments out of an old shaft that would fit over the condom.
I slip the condom over an old cartomizer tube, put about a wrap and a half of tape around the condom to make a tight fit for the golf club shaft tool, then push the tool over the condom. I expose a bit of the condom from under the shaft tool (the width I want for the o-ring) and use a utility knife to cut against the cartomizer tube, with the shaft tool as the straight-edge to .... the knife against.
I can make perfectly uniform-width o-rings almost every time.
