Up for grabs is a Revelate Designs Tangle Bag, size large for 55-60cm frames.  Made a bit of a mistake ordering this for my touring bike with downtube shifters, which don't jive too well with the bag, so this thing is brand new, never been used.  It's 21" long and 6" high and holds 350 cubic cm altogether in the two pockets.  

$60 shipped anywhere in the U.S., and Paypal would be the easiest means of transaction.

August 15th seems like a better time to be on the Kaibab Plateau, at least in regards to temps, but I wouldn't trust the monsoons to provide very good riding conditions anywhere.  No way to predict that sort of thing, but if at all possible I would try and delay your trip at least a month until conditions will likely be drier and temps slightly cooler.  I rode from UT to Payson late last September and I decided to pull the plug there because it was too hot to drop down into the desert.