I too read the Humanure Handbook. It's a great resource. Some people "flush" their composting toilet with a handful of Diatomaceous Earth. This keeps the compost sweet. Also, I'm wondering if the rainwater is such a good idea. Urine should provide more moisture than the compost needs. Commercial composting toilets often have heating elements to remove excess moisture. Also, if you can add a vent pipe which will thermosiphon air from the chamber, that might help. A small 12 volt computer fan could be tied to a small photovoltaic panel to increase the circulation, but that would be a more expensive proposition. Full disclosure; I have very little real life experience with composting toilets and most of that was sitting on one, so I'm one of those self-appointed armchair experts. As with all expurts, an ex is a has-been, and a spurt is a drip under pressure.
Also, you're Irish, so your poop is probably really stinky!
Just kidding!!! You shouldn't kid an Irishman unless you are one, and I are one. ;0)
It occurs to me that what some here are calling composting toilets may be closer to an old fashioned outhouse. This is not to disparage the humble outhouse. I spend half my time in the country, and half my time living with city utilities, so I'm in no position to cast judgement on a country mouse with an outhouse.
My point is that a proper composting toilet is different in several ways. Most importantly, a composting toilet isolates the waste from the ground water in some kind of containment. In a commercial unit,it's a fiberglass tank. In a home made one, it is usually a cast concrete tank. Secondly, there is some method of separating the liquids from the solids. This can be simple. the waste drops onto an inclined plane (ramp) with horizontal ridges. The liquids run down the ramp and collect in the tank, while the solids stay on the ramp and begin a slow slide toward the bottom. (Are any of you grossed out yet?) This gives the solids time to begin aerobic decomposition, while the liquids begin to evaporate.
There are several ways to speed the process. Air flow can be increased by adding a stack or chimney which carries air from the enclosure up and out. a breeze blowing across the top of the stack will draw air out of the stack. This has another advantage; the air getting drawn down into the tank at the toilet seat will keep any odor in the toilet. Air flow thru the stack can be increased by extending the stack higher and painting it black. It becomes a solar heater and the hot air rises. Another method is to add a small fan (like a 12v computer fan) to the stack.The air flow will enhance evaporation and decomposition.
Evaporation can also be increased by adding wicking (capillary action) or more commonly, by heating the liquid portion of the waste. If electricity is available, this is usually a small electric element. If a solar heater is to be used, some kind of circulation system is required, since the waste in the tank is the lowest point in the system.
I'm currently working on a workshop in the woods, and I plan to build a composting toilet in an outhouse. If I do, I'll photograph the building process and post the pics.