Have you considered using rubber backed curtain fabric. It is waterproofed - with the rubber backing - and has a reasonable amount of "body" to it.
I made a "man bag" for hubby by "googling" around and took what I considered to be the best of about 3 or 4 patterns that I found on the net and consolidated them into 1 taking into consideration what he wanted.
Good luck with the bag and don't forget piccies please.