You are going to want it to be about 10-12" high because you want the MHs to be at least 8" off the water for heat reasons. I just built a canopy for my 75 and I amsde it so that the wood is the exact size of the rim of the tank so it just sits on top with small boards that come down to keep it from sliding off. You will want to put a fan on one end as well and a vent on the other end. I have a fan on each end of mine blowing in the same direction but for a 30 gal you should be fine with just one. Hope thats a good start