I got tired of tripping over spools of filament in my office. I borrowed the concept from a design I saw (http://www.thingiverse.com/thing:51635) but made it lighter and faster to print. And prettier too. :)
The rod is a SS 3/8in 24in long. (you can get rods and nuts from Home Depot/Lowes etc. or online at http://www.McMaster.com)
I simulated the tower in Solidworks (files attached) and it should be able to hold up to ~100lbs total between the two towers.
I recently added a second tier to mine and it works great tool (http://www.thingiverse.com/thing:94691)
Print a left and right of each tower. You will have to orient it tabs down on your build plate.
The towers come with tabs on them to prevent curl. You will need to cut/snap them off. I found I needed to use dykes to do it. I used 5 shells, 10% infill, 0.34mm height.
I printed some spindles so the spools roll better on the threaded rod (the red bits). I printed them five on a plate, you can too. I printed them in the vertical so you will need to flip accordingly too.