I've lived for about 20 years with composting toilets that are just two chambers with a single room on top. Joe Jenkins would call them mouldering toilets rather than composting toilets, because we don't touch or mix or aerate the manure until a year later.
We make our chambers about 3 or 4 feet wide, basically just wide
enough for the door. The length can vary as much as you want. The height is about 8 feet, and in any case in my experience has to be high enough so that you have a full sized door. If the access door is lower, then it is unpleasant to go inside and empty it. If the length of the chamber were very narrow so that you could empty it while standing outside, that would be an option. But it might fill up in less than a year, depending on number of users.
Our school happens to be on a slope, so we just built the toilets into the slope, so that you enter from the upper side, and remove the manure from downhill. At my house, I designed it attached to the house. Downstairs, the manure chambers are separated from the wall of the house by a gap of 2 feet. Upstairs, the room is attached to the upstairs. But in Ladakh, the modern-traditional design is to have the toilet out in the garden, separate from the house, with steps up the front of the toilet. But the traditional Ladakhi method doesn't have two chambers, and sometimes doesn't have a door and lower chamber tall enough for comfort during removal.
The lower chamber is made of stone here, as people say that contact with the manure would "rot" concrete. The upper room is made of whatever you like; in our location, adobe bricks is normal. At my house, the contractor mistakenly put nice smooth mud plaster on the outer walls of the lower chamber, to match the rest of the house. After a year and a half of occupancy, the manure moisture is soaking through and making the bottom part of the external plaster look like hell. I wish he'd left it plain stone masonry.
In principle we empty one chamber every late winter and leave the pile just outside, with the dry top stuff in the bottom. Then we use the newly empty chamber and close off the full chamber for a year until the following late winter. We have more people using the toilets full time than the amount of food out gardens produce, so we tend to have excess manure, which means we can leave it sitting outside for an additional year. However many other factors often interfere with this schedule. If a chamber gets too full too soon we may have to empty its partner chamber before it has been closed for a year. Or if a chamber is not at all full after a year, we sometimes just leave it for another year, though I think everything works better and composts better if we do move it out after one year.
Occasionally, a chamber will be anaerobic and stinky in the bottom. My theory is that only happens when the pile got too high, and the weight of it pushed out all the air from the bottom of the pile. We've tried perforated pipes in the bottom but I don't think they make a big difference.
We use various materials for cover material, and keep a shovel in the users room, and a sign reminding users to throw a shovel full of cover material after each use.
Wood shavings are some of the only free biomass available in our region. In the early years I remember some of these coming out in the two-year-old compost taken to the fields. But in later years I didn't see them except in the top of the one-year-old compost during removal. Maybe the microbiome in there evolved to deal with them. Sawdust is purchased for fuel here so only the shavings are free.
Autumn leaves are good in winter, but as spring comes on they do not prevent flies and smell.
Garden soil is the traditional material in our region. It's the best for coverage, but I assume it makes less good compost, and is heavy to carry both coming and going. Sometimes we brought powdered clay from nearby because our gardens were a bit too sandy.
Cow shed scrapings are also traditional, but we found that combined with the high population in our school, they
led to smells and flies. With a much smaller population that might be great. Or as a small percentage of the material.
We don't burn much wood so we had almost no ashes to add. Too much ashes could be a problem, but a small amount mixed in is probably great for the compost.
A mix will probably give the best compost, and you can use whatever is free or available at that season.
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