Triumph part 57-0948 works fine
I got mine on eBay for £2.88 each!
They are a complete nightmare to fit mind. It's a hard job to compress the square section o ring so it doesn't get chopped off. I ended up making a sort of small version of a piston ring compressor from a hose clamp and strip of copper.
From this experience I too would recommend a standard o ring as long as it's chunky enough.
Ps - mine is brass same as the non o ring one and not sink alloy as I'd expected.
Cheers raymond