Many plastics have a good strength to weight ratio, but lack durability to such things as abrasion or chafing, and UV light. In my opinion, the key to successful use is not demanding something out of the plastic it is poor at doing. Laying down a rubber mat, removing any surface imperfections in the bilge, floors, or stringers, and avoiding oxidizers are a few that come to mind.
I can assure you that one 1/2" long quarter-twenty bolt under your tank will make it's presence known, probably sooner than later. Extra thickness may make it more later, but no less inevitable. Mounting/preperation is everything.
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.