Depends on how large you want.
I adapted a Joe Rocket magnet tank bag I had saved. Probably 8 years old. Kinda small but does have 3 zipper pockets.
I removed the magnets, folded and cross stitched the flaps under the rear pad. It's still solid after 24,000 miles and 3 years of riding. There's no metal buckles, straps nor tabs to fuss with. It stays attached to the pad not the frame.