To do a proper repair, one that does not depend on luck, you first need to determine what type of plastic your dealing with. Here are the common ones:
Type 1.
True fibergalss
Usually only used on watercraft anymore. Rough on the inside, with visible "hairs" (strands of fiberglass). This is typical fiberglass. you can use fiberglass cloth & resin, or cloth & epoxy. I recommend the later.
Type 2.
SMC
This is what is used on all hard, (inflexible), painted plastic parts on cars today. Smooth on the inside with a swirl effect in the unpainted areas, and barely detectable fiberglass strands inside. Rough up areas of repair, and use SMC repair products, (similar to bondo), or epoxy, and fiberglass cloth, (on the inside). Bondo can be used, but only for cosmetic repairs.
Type 3.
Polypropylene
Flexible, smooth, and except in automotive applications, usually unpainted on both sides, cast in color, with color all the way through, smooth or slightly textured on inside. These are the most difficult to repair because of oils used to prevent sticking during the molding process. The product needs to be treated after initial scuffing, sanding of immediate repair area, to remove release agents, or repair material will not stick. Passing a lit propane torch over the area several times usually is the quickest way to remove the oils. DO NOT HEAT the plastic beyond 400F, damage will start to occur. Repair with flexible repair epoxy, and back up the inside with fiberglass cloth or sheet metal, and epoxy. If painting previously unpainted plastic, the areas being painted need to be treated and a special adhesion promoter, along with flexible primers, and finishes.
Type 4.
PVC, ABS
Just like the pipe, PVC is usually white, ABS is usually black. These are commonly, and easily painted. Rigid plastic, softens when being wiped with a solvent. After scuffing up the repair area, use rigid plastic repair epoxy, and use fiberglass cloth, and epoxy on the inside to back up.
Since most of the professional plastic repair epoxies require a special very expensive gun to mix them, you can go to NAPA and other parts stores, and get a gas tank repair kit that contains epoxy and fiberglass cloth for around $12. These work great for doing small home plastic repairs, they also work great on gas tanks.
Consider taking the piece to a local body shop and get a professional assessment, and price for a repair. We often did these jobs as a customer courtesy in lieu of future business.