You can knot the strap. That’s what I did with my scooter when I had one. I didn’t like the knot at first but it grew on me. I took the bag all over the place & no one ever cared or noticed that there was a knot. Here’s the only pic I seem to have of the knot:
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This is probably the only way unless you permanently get the strap shortened since it’s one tubular piece of leather. Sorry
Eta: forgot to mention it, but in my pic the knot is done loosely because I took the picture as an example for when I listed the bag. You can tighten the knot which makes it smaller than what’s shown by pulling it upwards if that makes sense.