whats your price range for the amp? 500rms is kinda low for those subs but it will still be pounding. You could easily get by with 1000 rms with a stock electrical and the BIG 3.
here's the website for fisher customs Fisher Customs - Products. check out his prices, they are really good plus you could get him to customize the box for you if you want to.
theres people are here that make custom boxes at pretty good prices. get in contact with one of them. I dont know if bjfisher is still on here but his boxes are very nice! I had one built from him and i couldnt be any more happier