I have something similar, a homemade version based off an
army surplus map bag I bought for $10.
I sewed some extra D-rings on the back. It already has two at the top, I added two on each side, and two on the bottom. With a pair of toe-straps I can mount it as a saddle-bag (top D-rings to saddle rails, bottom D-rings to seatpost), handlebar bag (top D-rings to handlebars, bottom D-rings to front rack), and it makes a passable pannier (use your imagination). With about 10 feet of nylon webbing and a few hooks, It can also be worn as a backpack, messenger bag (with 3rd leg) or shoulder bag (man purse).