Rory and I built this composting outhouse earlier this spring. It's a wheelie bin system with urine diversion. It's a double envelope structure, so the fumes from the wheelie bin draft upward and out through the hollow walls, taking advantage of both the structure's chimney effect as well as some glazing to give the updraft some solar oomph. Works great, no smell.
Also, a few words on the business end of this thing.
The toilet seat sits atop a blue barrel that slides down into the wheelie bin. Within the hollow space of the floor, there are perforations in the blue barrell to allow the fumes to flow out and up through the walls. See the drawing for a better idea about what's going on.
With the double envelope, the most important thing is that it's sealed up both inside and out, so that there's only one way for the air to flow: from the poop, through the barrel perforations, through the floor, up through the walls past the glazing, and finally, out the vents. If you happen to be smoking while you're taking a poop, you can gently exhale the smoke above the toilet seat and watch it get sucked down between your legs.
The walls are framed up using some arched roof trusses from an old trailer house, so they're flat on the inside and curved a bit on the outside. The cross sectional area of the hollow wall down by the poop chute is about 6"x42" and up at the vents its down around 4"x42".... so a little shy of 2 square feet cross sectional area.
Decided to not do a traditional chimney because I think chimneys are kinda ugly. A chimney would have been much easier, but a less personally gratifying build.
UPDATE! Wheelie pooper works exactly as advertised!
I emptied out Can #1 after it had sat for 2 years. It had "cooked down" to about half its original volume and it resembled black crumbly potting soil. Very pleasant aroma (smelled like green bean casserole or mushroom gravy or something savory and delicious from Thanksgiving dinner). Not at all unpleasant to deal with. Mixed it into a hot compost pile.
It should be noted that each "#2" was sprinkled atop with a generous scoop of sawdust. I think all the sawdust helped dry things out and helped develop the crumbly texture.
It also should be noted that I drilled a vent hole under the rim of the can, which I think also helped dry out it all out.
You didn't happen to get any pictures of the finished product, did you? (Never thought when I was younger that I would be excitedly asking to see pictures of someones two year old poop!)
"Instead of Pay It Forward I prefer Plant It Forward" ~Howard Story / "God has cared for these trees, saved them from drought, disease, avalanches, and a thousand tempests and floods. But he cannot save them from fools." ~John Muir