You could start out taking a small paper bag apart and seeing if it could be constructed of fabric using the bag as a pattern. You'd be able to see if fat quarters would make that size bag, for example.
I'd certainly toss what I wanted into a paper bag and see if everything really fits.
A pillow case pattern might be a good construction method just downsize and square off the corners of the bottom and get some kind of pull string for closing the top of the bag.
Sounds like a great idea!
Jois