20 long or high?
I just laid my 20l out to the minimum space needed for the skimmer and return pump. That allowed for the maximum amount of fuge area ~ 12" of space.
Flow went from skimmer/influent -> fuge -> return. The baffles are between the fuge and return areas.
There is a calculator on the home page to figure how many gallons of room you will need for a power failure in the sump. On a 37 mine was roughly 5 gallons. I came down 4" on all my baffle heights to leave room for that 5 gallons.
If you search 20 gallon sump, you can see a bunch of builds on this site.