One of the most common plastics used on ATVs is HDPE (High-Density Polyethylene). You can get HDPE filler rods, or you can cut up something like a 5-gallon bucket or lid from Home Depot, Lowes, or Harbor Freight because they are usually HDPE as well. Of course the best match for material and color you can get is to "borrow" some plastic from somewhere else on the ATV that's not visible or necessary.