The pit toilet at our cabin fills with flies every summer — what actually works?

The vent is the main fix. Two supporting measures make a large difference alongside it:

Keep the seat closed. Not for smell alone — a closed lid means the only path into the pit is the vent, and the vent is screened. A self-closing lid is worth fitting because nobody remembers.

Add dry carbon material after use. Sawdust, wood shavings, dry leaves, peat. This is the composting-toilet principle and it does two things at once: it covers the surface so flies cannot lay on it, and it shifts the pit towards aerobic decomposition, which smells far less than the anaerobic kind. A bucket and a scoop beside the seat is all the infrastructure needed.

Between a vent and a cover material, most huts go from unusable in July to unremarkable.

Worth adding what to avoid, because the instinctive responses make it worse.

Do not pour disinfectant or bleach down there. It kills the organisms doing the decomposition, so the contents stop breaking down and start smelling worse. This is the single most common mistake.

Do not add lime routinely unless you know why you are doing it. It suppresses odour briefly and raises pH enough to slow composting.

Do not add water. Keeping the pit dry is most of the battle; a wet pit goes anaerobic and that is where the worst smells come from.

Also worth checking whether your flies are coming from the pit at all. If there is a compost heap, a bin, or a dead animal under the hut, you may be solving the wrong problem — a pit that has been vented and covered and still swarms is telling you the source is elsewhere.

The mechanism is airflow, and once you see it the fix is obvious: the flies are following the smell, and the smell is coming up through the seat into the hut.

The pit produces odour continuously. If the only route for that odour to escape is upward through the toilet opening, then the hut is the odour plume, and the flies are simply flying to the strongest concentration — which is exactly where you are sitting.

The fix is to give the odour a better route out, somewhere else. That is what a vent pipe does. A dark-coloured pipe running from the pit up past the roof heats in the sun, which drives an updraught, and the pit's air is pulled up the pipe instead of through the seat. Put fine mesh over the top so flies cannot use it as a door.

Done properly this transforms the hut. Odour drops enormously and the flies follow the odour to the top of the pipe, where the mesh stops them. It is the standard solution for exactly this problem and it is a one-afternoon job.

For the immediate term, before you build anything: a head net.

It costs almost nothing, packs to nothing, and completely removes the specific misery of flies going for your face. Not a solution, but it makes the situation tolerable while you sort out the vent, and every hut like this should have one hanging on a nail.