You could hide a small heater inside the hang on the back filter like aquaclear, or possibly new rio nano skimmer
http://www.toofishy.com/product.php?...0&cat=0&page=1
that is just about the same shape, i could be wrong on latter, but i will look at one of these rio nano's for my 6 gal nano. right now i just use my old ebo-jager stuck on the middle of my tank.