The simplest way to reduce outhouse odour is to properly exhaust the stank using natural convection. It’s an easy upgrade that you can do with 3″ or 4″ PVC sewer pipe and fittings that glue on with PVC cement.
Cut a hole through the back wall of the outhouse, close to ground level, and slide in a piece of pipe—the stack’s stink inlet. The pipe should protrude about halfway under the bench, right or left of the seat opening. Use a 90-degree PVC elbow to connect this inlet to a vertical pipe that extends two or three feet above the roof line. Any shorter and you won’t get much convection. Support the stack with blocking and galvanized hanger strapping. For that big-rig look and some rain protection, add a 45-degree elbow on top.
If you have electricity handy, consider adding a 4″ in-line duct fan to your vertical vent pipe. The one I installed plugs into a regular outlet and pulls air up the stack at a rate of 100 CFM, dramatically improving indoor air quality and outhouse odour. There are many types available, especially online, where these fans are aimed not at outhouses but indoor grow ops.