yo. you should make a custom little fixture for a 20W 50/50 screw in CFL (the "energy saver" fluorescent bulbs)
coralife makes a 50/50 cfl @ 10 and 20w... they are actually pretty dope especially because you get two different tubes, one tube is 10k the other true actinic.
Small Aquarium Lighting: Mini Compact Bulbs
they only seem to have the 10w, but the 20w is out there! just don't feel like searchin for it... i have it, my LFS ordered it for me.
my tank first set up this way... i took the tiny lil 15w t8 that came with it out ripped the guts out of the light fixture (just the plastic hood..) then put in some 6500k CFL's, cheap ass way to get like 80w of lighting on a 20g. (cost me about 15 bucks)
eventually swapped the 6500k's for 50/50's now running a 70w metal halide, still not as bright as those CFL's.... so i gotta make a canopy so i can add the CFL's back in.
then i just took some extension cord and you can get this little light socket (got mine at walmart for like 1$) that just has a plug on the opposite end, so its a tiny piece like the size of a walnut, that has a light socket on one end, and the other end is a plug, so basically you can plug a bulb directly into a socket.... so just take the halogen light fixture off completely, and stick that sucker up under the canopy.
really simple...