I would do something very similar to a JBJ or Oceanic bio-cube. You could get your 20 gallon tank and cut acrylic to portion off a section of the back. Use a dremel tool and cut some notches out of the far right corner for a surface skimmer. Section three "blocks" behind the acrylic and there you go. I would do the sectioning similar to a sump, under, over, under, and then through the return pump.
I would put a small skimmer in there some where as well. I would send the water from the first chamber (surface skimmer) through some LR rubble or foam. Second chamber could be a small fuge with algae and a sand bed if you thought it out (ebay leds on 24/7). Third would be your return.