There's another one, the Jettender, out of Croatia. No distributor in the US but he says they would ship one. I'm thinking not the same level of sophistication as the Williams but it does look like a hellovalotof fun.
I had been told that one of the problems with Rotomolded plastic parts is that they tend to sag if not stored properly. If you used weaver davits and left it on its side too long, it would sag.