How about an adjustable strap w/clip like they use on backpacks.
Take a strip of nylon backpack material and clips, connect 1 side to the foot pedal and the other to the front of the bag. The one on the foot pedal could be a simple wrap around or bolted on.
When you mount the bag, you click the backpack clips, then tug the strap to make them snug.
I use this on my tank bag after the sipper broke and it allows me to instantly adjust the size with a tug on the straps (not pretty, but functional).