I had to buy more MDF than I needed, because my lumberyard only stocks 4' x 8' sheets, so I actually have extra material. Also, the great people at the fastening supply store I bought the hardware at gave me 6' all-thread rods instead of 3', which means I can build a second equally sized rack for the same amount of money.
You have probably seen Flexy Racks all around the web. My inspiration was a dual rack found at http://www.tnt-audio.com/clinica/flexye.html. Full credit to the folks at TNT Audio for the great idea.
I decided to post clear instructions for a dual rack with measurements, since it's mostly a guessing game, and I love clear instructions.
Disclaimer: I make no warranties, express or implied, about the process laid out here. Always use safety equipment, don't hurt yourself, etc. Most of all, have fun with your cool new rack. :)