Have you considered a par38 bulb? They screw right into a regular lamp socket. I had two of them over my 20g and it looked pretty nice.
here is an example of what I'm talking about
Orphek PR-25 (12 LEDs) Dimmable LED Pendants
You can find cheaper options on eBay and ones that aren't dimmable are cheaper as well.
I was in the same boat when i first set up my 4g and then again with my 20g where i didn't wanna drop $300+ on a fixture. The single par38 over my 4g was a bit too much so two might work for you.