Ana-White.com posted a set of instructions on how to build these lovely shelves.
As the instructions note, it would be pretty easy to alter the dimensions of this shelf to meet your own personal space requirements. They may cost a little more than the Sauder ones from Walmart, but they look like they would support a heck of a lot more weight.
Check out the link below for a complete set of instructions.
Link: Ana-White.com