Well my enclsoure is 30 wide so I have the hood that is 30 and the bulb is 24.The bulb is made for that size hood cause there is plastic on each end that takes up 3 inches on each side. I guess if you got the 30 it might hangover on the sides of the cage or you could angle it diagonally. Not sure if that answers your question or not. Get the 5.0 tube for that size cage.
I have the reptibreeze XL and have a reptisun 10.0 18" bulb - it sits nicely on the top of the cage and I have rigged up a reflector (with a shoe box lid and some tin foil) which fits perfectly over the top