The thing I don't like about the magnetic closures is that it's not a positive connection. If you snag the flap on something, it can pull it open. I like a button & loop closure or something that locks it down a little better.

That said, a hunting style sporran does seem easiest. Even if it has a simple one-handed closure system like you see on change purses (two rods with knobs on the end that sort of 'lock' past each other), that would likely be enough to keep it closed.