Thinking I should add some mushrooms, I got out my Fimo clay (after two years, it was still easy to work with) and made some little 'rooms with a toothpick inside each for ease in sticking them into the terrarium dirt.
Looking at Sara Midda's mushrooms as examples, here was the process.
A wedding present from 1990 was used as a topper for the terrarium. Yes, it is a glass cake cover and a very heavy one at that. There will be no mushroom escape from this device! A 9 inch cake pan was the base of the terrarium, painted green with acrylic paint. Then I found a mirror with a turquoise frame, about 10 inches round. That is the holder and base of the entire terrarium.
To read more about how to consruct the layers of a terrarium, this post from March 2008 describes the process in detail. Who knows, I might make a few more!