actually, grocery bags are made of low density polyethylene i believe. the only problem i can see happening with a polymer like polypropylene is that it has different elastic modulus than woods. the problem with polymers is that, depending on slight variations in chemistry, the mechanical properties like elastic modulus change. for a sub box, my guess is a high elastic modulus would be best, so, i guess it all depends how stiff this polypropylene is. i haven't seen it yet.i have no idea....polypropylene tends to build up quite a bit of static though (think plastic grocery bags and the slides at Chuck-E-Cheese). I don't know whether that would make a big electrical impact on the sub or not
http://en.wikipedia.org/wiki/PolypropyleneWhat is polypropylene??