We have built a "treebog" which is probably the simplest form of compost loo, allbeit slightly more tricky to build. Although no need for it to be fancy. No need to empty it and for hutting use I very much doubt it would ever fill up! The concept is simple: raised toilet seat with area underneath where everything falls on the ground, screened with straw or something so you don't see it. Plant willows all around the loo and they soak up all the nutrients and kind of empty it! You can coppice the willows every year or two. Plenty of articles on the web.