This is a Rubbermaid(tm) Clear plastic container with 1 3/4" of perlite on the bottom and 1/4" of water. It's holding humidity well, or at least appears to be.

Next, this is something that I'm just playing with. I have another Rubbermaid(tm) container with a hole drilled in to the top for the fish tank bubbler hose. The hose is fed into a jar of H202 heated with a small submersible aquarium heater.

And finally, the ultimate killer super easy terrarium....the dual chambered PF aquarium.
This is a ten gallon fish tank. I had a piece of plexiglass cut to 15 x 18 " to act as a splash guard for the shrooms. The plexi-glass shield goes in the tank at an angle making shelter for the birthed cakes. The top is simply covered with plastic wrap. the cakes are on jar lids.

Hope they help!