I no you asked not to give you info on making yourself, but consider it, it took me less than an hour to do, and cost me no more than a tenner. it's so simple, just look at the result (Taken from dj_sim) which is similar
Just need to cut a sheet of mdf/ply into shape, cut round your sub box, nail two bits of wood to the bottom to the desired height, cover in carpet. It really is so simple.
If you really really dont have the time to spare.... Ebay do have some premade ones, but sometimes they go for £250 plus....