Maybe I should build an led grow light? Anyone know any good methods?
You could do two Cree CXA 3070 modules. The way I would do it, is run each @ 37v x 1.4amps = 52 watts. About $65 or so for each module. Then you can add some reflectors from LEDIL.
Otherwise, LED's for such a small setup will struggle to penetrate @3'. Hell anything will, those Kessil are nice, but 36w's will yield you just over an ounce from six plants 1gm/w, if you can do better, why are you on here
A small HPS needs crazy amounts of venting, CFL's would be a waste of time and effort for that amount of penetration but a DIY led setup could give you the most potential and one of the more cheaper, but powerful and tried and true methods.
Especially with lenses for added penetration. Efficiency, less heat and PAR, tough to beat that combo.
I have a cab that is roughly about the same dimensions and I only use about half of it with LED's because I always LST in a scrog style. It's just not worth any light source to penetrate 3 feet with such a small footprint. Shit gets in the way. Comes out lovely as a 70w LED grow though.