He's talking about getting a regular cube made out of wood.
Don't be afraid of fiberglass. It's not difficult to work with at all (just takes patients) and is very strong when done correctly. Don't use the crap at Walmart or Kmart for the project though, you'll spend tons buying their little cans of resin and little packages of glass and cloth. There is many places where you can find bulk fiberglass materials, especially if you talk nice to the right people. Stereo shops, marinas, boat repair places, auto body shops, places that repair canoes/kayaks, fiberglass retailers, etc. Even if some places can't sell you the material they can always point you in the direction of someone that can. Almost no one is going to tell you to get it on the internet or through mail-order, the hazardous shipping charges are too much, there's always some place local.
There is many different ways to lay up fiberglass, I'm not even going to begin to explain here because it could take 3 pages, no problem. But if you take your time and do it right fiberglass will be stronger than an MDF box. Using thick mat will yeild the fastest thickness buildup and therefor more strength with less work.
Check out Tempe's sticky up top and dig around in there for a while, there are some good fiberglass boxes to check out.
One more thing, you can make a box for your sub by glassing over the original panel. Who care's if it's a little bigger? Now if you want to exactly reproduce the existing panel you can't do it that way, but for your application I don't see it to be any sort of problem. If you have more specific questions ask away, I can probably help.
