Corbin (USA) has a solo seat called the "gunfighter" that is suppose to fit '71 OIF Triumphs. They go for about $300 US. Every one in a while you'll find one on Ebay, but for the prices they end up bidding it would be better to get a new one.
I was considering one for my '71 Tiger and emailed the guy at Corbin about the fit to the '71 frame. He said there wouldn't be a problem.
Don't know if that's the look you're looking for but you can go to their website and see a photo of it. You can also get a used seat (to save the original) and redo the foam to the shape you want and get it recovered. I looked into that also, but a Corbin was cheaper in the longrun.
Mike