If you are repairing a broken HPDE or other thermoplastic part, then welding is a good path IMO. However, if you want to make a skid plate from scratch, I would consider taking a piece of thin plywood as a base to make one out of fiberglass. The basic idea is to cover the plywood in epoxy and fiberglass cloth. Curves, etc. for the base plate can be made by adding molding material, such as modeling clay, and layering the fiberglass cloth on the mold. While it will take more time than welding, repairs can be made to restore the plate to original condition using the same technique.
However, learning to weld plastic would be more fun. Harbor freight sells an inexpensive hot air welder. Good luck!