How big a cross section are you looking for? Commercial 1/4" o-rings are available up to 26" ID. Unfortunately that's about 15% stretch to get to 30", not recommended. I wouldn't wouldn't recommend adhesive bonding for anything but a temporary fix. Vulcanized aren't bad if there are no temperature extremes and you keep the stretch to a minimum.

Its a long shot but sizes like this may also be available in larger cross sections (1/2 to 3/4") from oilfied elastomer suppliers. Stewart & Stevenson www.ssss.com is one supplier that comes to mind. Also if you need more than one or two, o-ring molds are 'reasonably' cheap to have made.

You dont mention if this O ring is for a static or dynamic seal.. In static seals the superglued ones can be ok, but I try to avoid them in moving applications.

These people have a 30" O ring listed

http://www.applerubber.com/include/pdf/oringsiz.pdf

TimD5,

Try http://www.apandg.com/vulcanizing.html

American Packing and Gasket can make the ring you need, and they might even have it in stock.

Regards,

Gunnar