McMaster-Carr sells o-ring in cord stock. From memory, the stock seal is about 3mm (1/8") in diameter. McMaster offers a round o-ring cord stock, P/N 9864K29, which think this will work. Its made from soft neoprene and should be oil-gas resistant and soft enough to deform as the halves are assembled.
I've ordered a 3' length. I'll report back on my success/failure.

http://www.mcmaster.com/#9864k29