You might look at sign manufacturers they usually use all kinds of plastic substrates (places similar to http://www.proveer.com/Proveer/userHome.action). You might also look at alupanel... http://www.multipaneluk.co.uk/ it is quite unbelievable for weight and stiffness. It would make one heck of a rack body and shelves! It isn't super cheap but less than poly carb.