I love the diaper totes you can find on Ebay. If you search there you can get and idea of what I mean. I am making one like that for a shower in October.
I used this site for basic building pictures. They have great info on making the bottom of the bag square.
http://www.bundlesoflove.org/diaperpattern.htm
I used this freeware program to design the basic pattern. A lot of the Ebay listings include demensions, so that can give you a good sarting place. I used the clutch bag set to the size I want. I plan on cutting three times the fabric in contrasting colors one for the out side, one for the lining, and one for the pockets. I think it will also need some heavy interfacing to help it keep it's shape!
http://www.wildginger.com/products/wildthings.htm