Shrink wrap doesn't do well in the sun for very long. UV will break it down.
+1 on bolting it to the fender. Get a cheapie plastic kitchen cutting board and cut it to fit the bottom of your bag. Using bolts and "fender" washers,
fasten it thru the cutting board, bag and fender with some nylock nuts to finish it off. FWIW, I have not yet lost a fender bag that is held on by it's little straps. YMMV.
1991 Honda XR628R
1978 Yamaha XT500
1965 Hodaka Ace 90