I spread a thin layer of fibreglass compound on the inside, lined with fibreglass matting while it was still tacky then another thin layer of compound over that. The idea being that it keeps the weight at a bare minimum but even if you drop your lid from a great height the matting sandwich should stop it breaking into lots of pieces. One layer of resin will be fine on the outside and I can strongly recommend 'keying' the surface or the bondo may just fall off! If you have any other questions give me a shout, always willing to help! Check out my bucket WIP link below, hope it helps a little.