The buildings are built into the game already anyway so maybe topping the patrol size at 10 or something and adding a Waystation building or whatever that increases the size by 5 every level. A 25 troop patrol would be able to chase off most bandit groups and if it got wiped out it would probably be by something worth looking into anyway. The idea isnt to shut down the bandits and looters just limit them a bit and give part of the garrison something useful to do.