Andy, I think you have the right idea (if by "plastic" you mean CPE pan material), but constructing a shower pan that won't leak involves a number of details that are not easy to convey in this medium. You can get a quick overview on the net at www.pascospecialty.com, but for complete details, get a copy of Setting Tile by Michael Bryne and his "Floors" video to see it done. Both can be had from www.taunton.com.