Pex,
I just mounted some cheap Chinese hard bags made of ABS plastic. I used a piece of stainless steel sheet metal (almost 1/8" thick) and cut it 14" x 10" and trimmed off the rear with the top being 7" wide. 1/8" aluminum would have been perfect - but then I priced aluminum sheet. I used a break (brake?) to bend the last 4" on the bottom end 90°. Then I used some 3/8" SS tubing as a spacer to move my bracket out from the fender and hold it parallel. The front spacer was 5/8" long and the rear was 7/8" long. I had to buy longer SS M8 bolts at Lowe's (increased length by 1/2").
This will work with soft bags and help stabilize them and keep shape. And 14" x 12" will fit fine (almost too small). I was looking at some 20L x 14H bags 8" thick, they would fit too, just had to mount them right.
