(I'm only in the process of building my first sump/fw fuge.)
Here are instructions that helped me: [link]http://reefcentral.com/forums/showthread.php?s=&threadid=445481[/link]. I encourage you to read the entire thread, as a lot of people think this is a bad idea. I only used a 7134 bit when drilling
mine. If you do not want to drill, there are many
diy overflow designs in the archive.
Unless you only want to hide your equipment, consider using a sump larger than 5.5g, since you'll probably only add ~3-4g of usable water volume to your 20g tank. How about a rebermaid container sump (ie: 20g or 33g trashcan, 12g rubermaid container) into the 5g as a
LR/algae/plant fuge? You could use your current canister anywhere you want (main tank, sump, fuge) since its all the same water. Or you could use the cannister as a redundant filter to the main tank - you pull the return pump's power, instant
qt tank in the 5g and the main tank only loses sump's water volume. Just a thought.