Summary

The Diameter in the bottom of the bowl is 5.8 inches.

The plunger sits on raised half-spheres to allow drying of poo-water for minimum long-term odor.

I made this a while ago because cheap plunger + this was about $4 and a plunger with a receptacle was about $10 and I needed 2.

Update (11-14-2016): I added a lite version that uses 40.6% as much plastic for those who need a plunger bowl more quickly :)