Although the skimmers suggested are good/great models, you may find them a little large for a small sump (20 gal) with height restrictions.
The EuroReef RS135 has a 7" x 8" base x 24" tall . You might want to look at the Tunze DOC 9010/9015 models. They seem to be the smallest skimmers (4.5"x5" footprint x 17 tall) in this class. There are a couple of people in this forum that are running them and seem to be very happy with them They also fall in your price range. Have heard that the ratings are generous, probably done with a light bio-load.
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-captainhemo
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