With that size tank you may be able to get away without a sump/skimmer and a low fish load with just a few power heads.
Have you looked at the overflow kits from glass-holes.com ? Their overflow boxes are fairly narrow and wouldn't take up much room in a 20L.
You say some kind of sump or something, but what size are we talking about? 10g? 15g? Another 20L? And a skimmer, return pump, heater, baffles are all gonna fit down there? On second thought, it might be easier (and simpler) to do without one.
How about a AquaClear 110 as a HOB
sump/fuge? We have a 29g at work with an AC
110 on it, PCs over top, softies, one or two LPS
+ snails + hermits, a moderate fish load, and things are thriving.