That height really limits the options. Here's a few that would work.
21.8" high
http://www.aquacave.com/skimz-sm203-monzter-dc-series-in-sump-protein-skimmer-with-dc-pump.html
http://www.aquacave.com/nyos-quantum-160-protein-skimmer.html
http://www.aquariumspecialty.com/ati-powercone-200-i-series-skimmer-with-controllable-pump