Here is an easy method for anyone who is short on space in their terrariums, or is looking for a low-cost, easy terrarium:
My friend likes to use 2 gallon Ziplock storage bags (you might have to visit a few stores before you find them, but they are available.)
Then- use 9er's idea of casing the cakes in a 1 gallon plastic jug with the top part cut off, or any type of shallow, easy to find, cheap plastic container that will fit in the bottom of the ziplock bag, and still leaving plenty of room around the edge of the the container- such as tupperware from a thrift store, etc..
Place the container in the ziplock bag and seal well, if possible blowing in extra air from a hepa filter to exchange the air and also to puff up the bag a bit. If you don't have a HEPA filter, just make sure that the area is sterile, and just try to capture as much air in the bag as possible.
The bag should support itself, but if it doesn't you can rig a support system with a coat hanger or chopsticks, or anything else that you have laying about that is free/cheap (sterilize it before putting it in the bag, whatever it is).
Airing the terrarium out is easy- just unzip it and push the old air out, re-expand the bag and zip it shut, and the bag acts as it's own drip shield.