Were looking to install a single-piece, 48 shower stall. We need something with a base that does not flex (or flexes minimally) under 250 lbs. We are not interested in any type of tile. Based on your experience: Which material is best: fiberglass, acrylic, gel-coated, other? Is there a particular brand that is better than the rest? Should we pour a concrete pad? Id like to keep the price under $600 (excluding the door) if possible.