Plastic welding (when you know how) is a very stable joining method with strengths equal or greater than the base material.
Though the technique is somewhat different than gas welding or brazing the concept is there.
Countless tanks, fixtures, and structures are routinely fabricated from plastic and joined by hot gas welding. When done properly there is no concern for failure.
How well will a plastic skid plate function, I don't know. I think I'd call it a splash shield or stone guard and hope for the best. None the less it can be fabricated though it will most likely look bulky due to the material thickness required for strength.
I'd suggest you try here: http://www.usplastic.com/catalog/sea...=welder&page=1
They offer multiple units.
A few points for the actual welding. Cleanliness of the base material and gas (air or N2) is utmost critical. Your not going to get away with an oil type air compressor with excessive moisture mixed in. A diaphragm type compressor with a method of moisture removal is good start.