Well it's not a 5 gallon solution, but have you seen the 3 gallon mini cylinder aquariums that are marketed for freshwater? They were made of acrylic, so they're drillable. They have an undergravel filter with an airstone riser right in the middle, and they're lit by a LED wand from underneath. I had one for a betta for a while before I donated it to a young cousin as her first fish tank. Before I donated it I thought of some ways to convert it to saltwater: Drill a hole in the bottom middle of the tank, modify the air lift riser to act as an overflow (it was deliberately wide so you could see the LED lights through the bubbles) , and silicone the riser so it stays put.
These tanks were cheap as hell -- I think I got mine from ***** for less than $30.00. Might be worth looking into.