I would see about doing a BIG messenger bag. I don't think most diaper bags are "girly" patterns I think the pattern and colors of the fabric that they are made of make them "girly"!
this is a good tutorial
http://bluebirdbaby.typepad.com/phot...ial/index.html
the first page is a materials list, click on enter gallery for an AWESOME step by step how to! this pattern is super easy to change the size too!