Just thought I'd come back and give an update. The light is working beautifully so far. I ended up just putting one T8 strip in for now, and I think that's all I'll need. I have ludwigia, ruffled swords, anubias, and water sprite in the tanks. I know none of them are particularly light-demanding, but they are all growing nicely and look really healthy. They are mostly trimmings from my higher-light 54 gallon tank, but the ludwigia even seems to be keeping its reddish hues.
It was warming up the water at the top of the tank slightly (though the glass was only slightly warm), so I took the feet from my Finnex Ray II which I wasn't using and screwed them onto this one. I wasn't using the feet I mean, I am using the light fixture on my 54 gallon tank. It raises it up about an inch which is just enough to prevent it from heating the water. Cooling fan definitely not needed.
#1 is the tank the light is on. I don't know why I took the picture in the dark because you can't really see the light fixture. It looks nice on top though. The tank is a 20g long and home to bettas Roland, Ender, and Elise who is the little cellophane double tail plakat you can see in the rightmost division (Elise is my daughter's fish, so she named him after herself. She's 3.)
#2 is the light and the hood separately. I just got some silver drawer pulls to make little handles on top of the light fixture to make it easier to pick up and move around.
#3 is the inside of the painted gutter. I actually spaced it out so that 2 of the screws holding the handles on double as mounting screws for the light - it's one of those deals where it has holes on the back that you slide the screw head into and then slide to lock - knowwhatimean??
#4 is the light snapped into place. I made end caps out of pieces of corrugated plastic painted with the same black krylon I used on the gutter just glued into place, with a hole in one side for the light strip cord. I didn't want light to bleed out the ends.