Harbor Freight offers a couple of different Plastic Welding Kits.  One
has an adjustable temperature control which is desirable in that
different plastics melt at different temperatures.

Here's the adjustable one:

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=96464

You can also view the manual (.pdf) and see what you think.  I've never
used one.

Mike