I'll second that vote for Urethane Supply. I bought their KC Welder Pro, and it's made plastic welding fun. The results are nearly as strong as the base material you weld, and it's surprising how nice a finish you can get; take your time, and you can get your work to the ready-to-sand state.
Check out turbocad6's all-time classic thread on building a bezel for his FX35. Be sure to weld with good ventilation, though -- the fumes aren't just stinky, they're toxic.