I’ve had a few pairs of jeans where the fly comes down. So I’ve used several things to keep it up but not restrict me. The best one is women’s hair ties. Essentially this loops around the button, goes through the zipper and around the button again. When you undo the button the fly comes down automagically, when you do up the button the fly is done up automagically.
I was thinking you’d just buy them in bulk and repackage them and sell them at the counter in jeans stores.