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.
Quite pleased with it.