The main thing with soft bags is keeping them away from the wheel and chain. The Magna braces mentioned by someone else will work for this. For that matter, it isn't all that hard to make something yourself that will do the job. Even a simple T bracket made by welding or bolting 2 pieces of steel strap together will work.
If you very seldom use the bags, an easily removed brace, home made or otherwise will serve you well and save you money.
In a case like mine where the bags are used frequently, but not enough to justify hardmounted fiberglass bags, Easy Brackets are worth their price.
Since the Easy Bracket has a good size heavy metal backing, you really only need to add a reinforcement plate on the inside of the bags to make sure they hold up.
The lock assemblies are pretty hard to get at once the bracket is mounted, so the way to steal the bags would be to open them and remove the bolts. Tack welding the nuts to the bolts will prevent that. The bolts holding the studs to the fender brace are inaccessable with the bags in place, so it would be really hard to steal the bags by undoing them.
