Hello from Arizona and welcome to the Board!
Not exactly sure these will fit the bill, but ...
I found a changing pad that rolls up. The pattern is for sale but you can figure out the pattern by looking at the pictures and the site tells you how much fabric you need.
http://www.etsy.com/view_listing.php...ng_id=24001154
Would this work? It looks more in line with what you describe and could be adapted to your particular design idea.
http://www.cluckclucksew.com/2009/02...-tutorial.html
Have fun and please post a pic when you complete this project :D