Best place is Motorcycle parts shops, they sell a pack of 3 A3 sheets of 3M clear adhesive plastic, for around $20, it can also be bought in patterns and colours. It is by far the most resilient material i have found. Apply to the chassis, and use a sharp blade to remove all the cutouts and screw holes. I also apply 2 layers to the rear 1/3 of the chassis, as this is where it wears out quicker, once it starts wearing, remove the upper layer and apply a new piece.