Just measured my kit. With my bulb (other 55W bulbs can be a little longer) in the endcap the length is 22.25, the reflector is a hair under 4 inches wide and 2.5 inches tall with the spacers. It will be a very tight squeeze, and for sure there's no room for the ballast. You will have to mount the ballast on the outside of the fixture which is hazardous - exposed to the water and all. And even if you manage to fit it, it will get very hot.
I just recommend making your own enclousure. Oh, and a power saw would be very nice to have. Without one a regular handsaw might make the enclosure a little more difficult to build.