After -lots- of research I just bought this fixture (250W) from J&L for my 37 Cube 20x18x24. That might be an option for you?
HORIZONTAL METAL HALIDE/POWER COMPACT FLUORESCENT PENDANT
This pendent uses our parallel reflector to focus the light into the tank as well as supplying sockets and an electronic ballast for 2-28w Power Compacts. Mogul base metal halide up to a 400W bulb. The metal halide ballast is remote with an 8' cord. The pendent should be mounted with the bulb parallel to the front of the tank. The power cord comes out the side with eye bolts on the back to hang the fixture. It can also be attached to an existing hood. The pendent is made from powder coated aluminum. A 4' fan is included with a glass lens. The metal halide and fluorescent lamps can be independently controlled.
Dimensions: 12.5"W, 7.5"H, 16"L