6 inches is the most common. It provides the best access both for the installation of the strap, and later if you want to store things in the hull, sponge it out, etc.
Installed my hiking strap yesterday. This is a Starfish sailboat, definitely not the same construction methods as a sunfish.
Had one large block of foam that wrapped around the centerboard trunk and went up towards the bow. No foam in the rear behind the cock pit at all. There was only enough water to soak up half a small rag in the boat.