I helped my mate with the basic shed building element of a compost loo over the summer in Dorset.
They got a guy in to drop a 1.5 metre hole in the ground, which would get lined with something, maybe lime, and then some pebbles in the bottom for a bit of drainage, something like that.
And then the ex-garden shed, without the floor, gets built over it.
I think the main thing you need to try to do in terms of use is separate the wet and the dry, cos it's the moistness that makes unpleasant aromas.
Basically learn to separate pissing and shitting as discrete activities.
But apparently if there's only a few people using it, sparingly, this sort of method is pretty good, can last a fair while before you need to build another.