i have perspex boxes!! although bambam is correct, the sound quality is not perfect!
however, the boxes i have are 20mm thick (which you cant really buy or machine through normal processes, my brother owns a laser cutting company, so unless you know anyone in the industry - good luck!!!) they are also precision mitred joined (perfect joint really) and has been fixed in a way best described as platic welding (its not actually that but is more simple to say that!)
the boxes i have are probably the best built sub boxes ive ever seen, and although as mentioned above the acoustic qualities are not that great when the boxes are that well made, with such thick material (and heavy! with the subs in i can barely lift them!), they sound pretty damn good to me! oh yeah they look preety cool too!!
but to be honest i think if i were re doing mine now, i would probably buy ply boxes (better than MDF), and then somehow fibreglass them into a cool looking display, probably paint it bodycoloured or something, it would be a lot less hassle in the long run! good luck with it though!!