I used acrylic plastic on my CP and plan to use it on the sides of my cab to protect my side art, as you planned to do. I forget the brand name of the plastic but I'll get it if you want. It is quite cheap and versitile. I used a stronger, more expensive version for my CP, and was able to drill several dozen holes without hardly a crack (nothing visible). For a more general use, the cheaper version should suffice. I think it was called Plastikote but not exactly sure. I paid about $30USD for a sheet of the tough stuff (the lighter-dudy plastic is about half the cost), and it was enough for two tries of a CP at 14x40 inches, with a little to spare. I'd try that before filling in your sides with goo. Hope that helps!