made a few parcel shelfs in my time, and put the amp underneath, i find it better because you can hide the amp, no one has to know its there! first time i used this wood my dad gave me called marine ply, probably 10/12mm thick but very strong, have used MDF as well which is fine just be careful when u cut it cos the dust is not good for you. stick to around 12mm but make sure the speaker screws arent to big! measure it with the existing parcelshelf, or make a template out of cardboard, either work. buy some carpet from a proper car upholstry shop, dont be tempted by the stuff they sell at bunnings, trust me they told me it should easily survive in sun, it lasted about 3 weeks before it started disintergrating! to stick the carpet to the wood there is this glue stuff i cant remember its name but its at bunnings or similar, the people there should be able to help you out, and just secure the carpet with a couple of nails around the outside. oh yeah remember to cut your speaker holes first! apart from that its all pretty easy, and looks really good if u pick a nice colour of carpet! and ques just ask!