Sump for 20g

Yes and yes, though it may be more expensive than you thought.

You will need a return pump and an overflow box. (or drill a hole in your tank)
 
Do a search for 'amiracle overflow' They are pretty simple U-tube overflows that use to sell for under $40, Im sure theyve increased price a bit, but they should still be rather cheap.

Overflow boxes are fairly difficult to assemble yourself, and I wouldnt recommend it for someone who hasent worked with acrylic before.

Your canister filter can not be used as the return pump, the pumps in canisters are not made to operate against any significant head (restrictions) and a 3' pumping height will likely make the unit stall out. There are a good number of smaller pumps and powerheads that can easily pump 3'+ however, and generally dont cost more than $25.
 
Wouldnt work. As soon as the return pump stops the water would back siphon into the tank, and when the return pump started back up, the water would not overflow back into the sump. Leaving you with 5 gallons of water on the floor.
 
er, and when your return pump stops, your HOB filter keeps sucking in water until your sump overflows.
 
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