I built my own rectagle box out of wood and a lot of caulk with two seporate compartments in it and screwed on a set of 4" intake and output fixtures that I bought at Rockler. Here is the link to the component parts: http://www.rockler.com/product.cfm?page=25225 If you have a 4" collector then you probably should try to stay with 4" the whole way.

Alan