I think you are going to have a hard time finding a chiller to work on a tank that has no plumbing, although an Ice Probe might be worth investigating:
http://www.marinedepot.com/aquarium_chillers_coolworks_iceprobe.asp?ast=&key=">http://www.marinedepot.com/aquarium_chillers_coolworks_iceprobe.asp?ast=&key=</a>
However, do some of your own research on the Ice Probe since I've heard very mixed reviews on them... however, for a 20g it might do the trick if the temps aren't that serious.
How high are your temps getting? Can you just add a fan blowing over the top to control it?
You might also post this question in, or search for potential answers in, the Nano Reef discussion over at TRT:
[IMG]http://www.thereeftank.com/forums/forumdisplay.php?f=97">http://www.thereeftank.com/forums/forumdisplay.php?f=97</a>