Second Story on Cage

I can't seem to find online what I'm looking for and hoped one of you guys might be able to help.

I need to find a second story for my cage. It is 48x21 inches. What would be the best way to expand upward? Links greatly appreciated!

Use the cube system. Look at www.cavycages.com - it's a cavy site but has a section where other people have made them for rabbits, also try www.morfz.com/nic.html