Several years ago, as a Peace Corps volunteer in Paraguay, I experimented with VIPs, Ventilation-Improved Privies. The house we rented in a small village came with an odoriferous, fly-infested outhouse, as did all the houses in the village.
After moving in, I built a raised platform and installed a toilet seat, converting the privy from a squatter to a sitter. I bought a 6-foot section of thin-wall, 1 ½ -inch plastic pipe and (taking care not to plug the pipe with dirt) poked it through the dirt, from the outside into the pit area, so that it ended just under the platform on which the seat rested. Working from the sunny north side (Paraguay is south of the equator) of the building, I fastened the pipe to the outside wall vertically, being careful not to crimp it. Then, I painted the pipe flat black. As an indicator of air flow, I glued one end of a small piece of audio tape to the pipe outlet.
The next morning, I was outside the outhouse at sunrise. As the pipe heated, the piece of tape began to flutter, then stood straight up. As I watched, the flies left the pit and swarmed at the pipe outlet. After several days, all odor disappeared along with most of the flies. Later, as a further refinement, I wired a piece of screen over the top end of the pipe.
This simple outhouse ventilation system works for both squatters and sitters as long as a cover for the seat or hole is provided and kept closed when not in use. There should be a small opening of some sort just under the cover, which permits air to enter the pit area, flow up the pipe and sweep away any odors. Other pipe sizes may work just as well, but to promote rapid air flow I believe it pays to keep the pipe rather small.
I wonder why more outdoor toilets don't take advantage of this simple use of the power of the sun? The outdoor privies at rest stops as well as the Amish outhouses in our area all use 4-inch vent pipes, but located inside the structures. What would cause any air flow in these pipes is beyond me.