Cheap Fender Bag

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.

Cut a flat piece of plastic to fit into the bottom of the bag/cover.

(You use an old 5l oil container)

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 & plastic base, add washer, and nut.
Tighten it up and ride!