A one piece would have to be like a Lycoming prop shaft seal on an aircraft engine. They stretch it over the flange then install a garter spring. It is then glued into the engine case with a rubber cement (Pliobond). Anything else would have to be split then glued at the split.
I'm with Ol Ron the Best Graftite rear seals seam to be good stuff. They even come with a gauge for cutting and a razor knife to do the cutting.
I tried using the Buick seal recomended, 1983 3.8L, got it and decided that it looked like a stretch (difficult) to get it to work. If any one else has a note on this, I sure will be interested in it... Karl