You should be preparred that you may be between 30-40 for glass and silicone for your fuge though.
Start thinking about if you want your pump to sin inside you sump, or if you want to drill another bulkhead and put your pump outside your sump... this would give you more space to work in the sump.
I think you should plan for:
A polishing sponge to reduce air bubbles... and act as a mechanical filter.
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An area for at least a Kent or other brand float valve - I like Kent because theirs is mechanical, not the electronic switch type.
Enough room for your heater in the sump.
An area for your skimmer.
An area for media bags should you need to run, say, carbon.
I have been looking and I think the answer on that 20 gallon is to use more of the height. you sure you don't have room for a 29 gallon???
Here, look at this one from Madison... the MARS forum... scroll down to the custom sump and filter. now imagine that design modified to fit your 20 gallon.
http://reefcentral.com/forums/showthread.php?s=&postid=7776277#post7776277
We could even build in a small area for bio-balls or bio media for some biological or perhaps a small fuge. (fuge will take some time to think about - maybe that's where we pickup a small acrylic 5 gallon or something to "build" into your tank, and use excess water flow off your pump I think you should run at 700 gallons per hour at 5' head... that gives you 100-200 gPH to siphon off... and run thru a fuge or some other attachment that would help your setup.
So, what do you think...
Jason