I'm planning a 12g long as well and have been perplexed by the challenge of lighting this tank appropriately. Since I don't plan to do CO2, it is especially challenging.
I have found two LEDs that seem appropriate: FugeRay and MarineLand Double Bright LED. The Current Satelite Freshwater LED+ would probably work as well, but for the price it doesn't produce much light (iirc ~30 PAR at 12"). On such a shallow tank it would probably be fine, especially if you like the remote control and all of the features.
I am curious if a 24 or 30 inch fixture would adequately light the tank, as they are significantly less expensive than the 36 inch fixture. A 30 inch fixture would be missing 3 inches on each side, while the 24 would miss six inches. I could see the 30" ray being pretty acceptable, especially with reflection off of the glass. On the other hand, the marineland seems to place the LEDs about 6" apart anyway, so I wonder if the 24" wouldn't actually be adequate for medium light.
Another good option is 1-2 T5NO bulbs. In the end for a budget medium light setup, this is probably wiser than trying to stretch a too short LED fixture.