I'd get a prefab sealed box and port it to your needs over a prefab ported box.
Here's one in your price range.
http://cgi.ebay.com/DUAL-12-SEALED-3-4-MDF...id=p3286.c0.m14
Most prefabs really need some poly batting to combat the noise they make from being squares and cubes, if you ever play one without it you'll think it sounds like hell.
Just go to any chain fabric place and get some poly batting thats 1" thick, its a roll of poly and not poly fill. Prolly need around 14sq ft to cover all the surfaces and its pretty cheap so may as well get extra.