I went on a 5-day trip using this inexpensive setup. I think I paid $5 for the scrap of aluminum, and maybe $3 in stainless nuts and bolts.
Mark and drill 3 holes in the OEM luggage rack
Mount a heavy piece of aluminum using stainless steel bolts, flat washer, lock washers, and nylon lock nuts.
This is a 45L semi-waterproof duffle, and the bottom has a semi-rigid foam liner.
attached with a few strong bungee cords