[e46er]

you will need an overflow box of some kind to cut off a syphon if power goes out and when you shut off the return pump to do a water change

you could drill the bulkhead close to the top of the water level but you will need to make sure you have enough room in the sump to drain the display down probably 2 inches minimum leaving the 20 G sump to hold very little water
FWIW my 40 G sump only holds about 17-18 G of water and some sand and rock