I don't know where you're at but
if you query Amazon for "Little
Cherry" you will find a number of
backpacks the seem to meet your
criteria. The one thing I noticed
about them specifically was that
they are 'waterproof' not 'water
resistant.' Of course you do know
that nothing is totally
waterproof. Eventually water will
find a way to seep in, but this
just might be your ticket. Let us
know what you think.