Rotocast polyethylene seat shells outperform plywood and fiberglass traditionally used in upholstery applications, for all the following reasons:
No rotting
Economical
Guaranteed uniformity
Inserts will fit standard sliders and pedestal bases
Easy to upholster
Foam padding attaches directly to surface with adhesive
Ability to achieve compound curves
Smooth edges minimize fabric wear
Ultraviolet inhibited material
Polyethylene accepts standard upholstery staples