ive done lots of plastic repairs on bumpers, bodykits, etc. actually did a nissan side skirt yesterday.
ive done a repair on a front lip off another car that was broken into 5 pieces, that was my biggest repair.
i use a soldering iron. hold your join together, use the soldering iron to fuse it together, if its thin get some scrap plastic off another bumper or something and use it as filler, comes up a treat.
then fibreglass on the inside to give it strength, dont need much glass, just 3 thin layers.
on a big repair like if the parts in a few pieces, i cut strips of 1mm steel about 3inches long by about 10mm wide.
i place it across the break like a bandaid and tech screw it on from the inside of the part. (the sharp point of the screw will stick out on the good side of the bumper/skirt)
now weld your joins with solder iron then glass over your steel strips.
once all dry get the grinder and cut the screws off where they stick out and run a small layer of filler over the repair and presto! stronger then it was when it left the factory!!!