So, I just saw this on Deadspin.
It seems to me, this can be done cheaper by about $40.
Ikea has this backpack for about $11 or less (check sales). I have about 3 of these ugly things. But, these kind fold into little tiny sacks, as well.
To waterproof it, get some Scotchgard spray, less than $10, that can waterproof it. Done. Saved money over the good looking option that probably has better straps, too, as the straps aren’t nearly that nice, either.