with an aquarium filter...
I would use an internal powerhead type in a back corner with the flow running the back of the tank. just tilt it so it fits, the orientation doesn't matter. unless I had river fish then I'd run the current across the front. but pretty much any external canister or HOB that doesn't get obstructed by the stand would work too.
I had a 30G breeder that was wide and low and I used a HOB on the side of the tank (HOS filter) and it worked great. I had to cut down the glass top and add a lid support though.