If you grew up with the Honda XR series and the handy fender mounted tool kit, you know how hard it is to switch to a bike without one. Fortunately, there is a cheap solution.
Purchase a vinyl Bible Cover with zipper.
Measure and drill 4 holes in the flattest part of your rear fender.
Use an awl to punch 4 matching holes in the Bible cover.
Insert a round-head machine screws and large washers from the under side of the fender (round heads won't dig into your tire when you bottom out).
Run the screws through the holes in the Bible cover, add washer, and nut.
Tighten it up and ride!
I used one of these for 4 years of adventures with no damage and no loss of contents, though you might want to hold the zipper shut with a twist tie for extra security.
Keep it cheap!