The guy that painted my S3 had one that worked pretty good. Aerostitch sells it but is rather spendy if you only have a small job. It is basically a soldering iron with some special tips and different types of plastic rods for filler.
they use that same setup to weld commercial vinyl flooring seams. there is a good chance you might be able to rent one at a local flooring supply house or a tool rental place.
I have access to one at work. I've never used it, but it's been used to weld pretty critical prototype parts in washing machine tubs that have survived testing. I know we've got different rod materials based on the plastic substrate that we've had to weld.