You can buy a plastic welding kit. I use a soldering iron for plastic tank repair. We have a flat round disk that goes on an iron at work. It works great at keeping the welded area nice and flat. Pretty easy if you watch the material and how it is currently flowing when heating it up. I used zip ties, the big police issue size ones as filler rod/material. Most of the time you can heat and flow the plastic back together with no filler rod/material.
slomo