Best Box building material

Im going to build a box from specs given on the Elemental Design website

http://www.edesignaudio.com/im2/boxcuts/12%20rec%20vented.jpg

and my question is, what should I build the box out of? I know particle board is probably best, but even at 1/2" itl be really heavy. should I still go with the 1/2" particle board? or is there a better option?

and also, should I leave the wood bare? or does it matter if I cover it with any kind of carpeting? on my old boxes I used some gray indoor/outdoor carpeting (very thin). Does it matter? or is it better bare?

3/4 MDF, stronger than particle board and thicker. That particle board would most likely flex with any decent sub inside which is never good. And carpetting is just cosmetics, you can leave it bare if you want

you don't want to use 1/2 either MDF or particle board. Particle board will not flex though compared to the same thickness MDF. MDF is a denser material but the major difference is the finish. MDF with cut sand and route smooth...particle will not. Particle board will also break apart where MDF will not. Now if you are really concerned about weight you can make the enclosure out of birch plywood. Price is dbl of MDF...sometimes more but it also about half the weight and denser. There are different trypes of birch and different names. Home Depot calls sand ply and something else. Does not really matter you want to look for a plywood that looks smooth and finished...like indoor wood. You also want as many layers in the wood as you can get.
I build alot of light weight enclosures for guys with "race cars" and I build them out of fiberglass reinforced birch plywood.