I have a soldering iron type with a feed tube and an air welder, I prefer the "soldering iron" because i can smooth out the surface of the two pieces with the tip of it, less sanding involved. The air gun pretty much just melts the pieces so you can paste them together, you can use rods with both, but the speed tip (soldering iron) works best because it is more precise... If you combine the speed tip with a standard heat gun it is possible to mold the ABS into what ever shape you desire... A good way to make parts i have found is by fabricating a solid structure out of plaster or wood, then heating up the ABS with a heat gun allowing it to form over your solid structure, then using the speed tip to correct imperfections and piece it together. Make sense at all? Hope i could help.
I have the nicer Harbor Freight plastic welder that is about $70 and I use it to make custom PVC cold air intakes for a a lot of rides and it works great. I also do a lot of single DIN to double DIN conversions and normally use glue and lexan to fill in my gaps. I tried to use my plastic welder once and that almost didn't work out for me. I have went back to using glue and clamps ever since.
However, the soldering iron sounds like it would work better.
From the people I've talked to at a local plastics fabrication place, the hot air welders aren't really designed to work with the thin/small areas like a bezel. It's pretty much the same as with welding... It would be like cranking the amperage way up on an industrial unit, and trying to tack weld body panels... You're just going to blow holes, warp, and FUBAR everything around. For an aesthetic application like bezel modifications, the soldering iron style is perfect.