the filters would be fine without a cover.
you could look on craigslist or on the forums for used fixtures to fit that tank. it's easy and affordable to find something that will work. you could light a 12" deep tank with just about anything. as a matter of fact, if you kept the plants in the area just under the lamps, your fixture would work fine.
it's all about distance from the bulb, and the amount of lumens/lux, and PAR. not the gallons v.s. watts.