>>Yeah, the substrate is supposed to be about 1/3 the volume of the terrarium, but much depends on A) the shape of the container and B) what you want to grow. Obviously lichens don't really need much soil layer and moss will be happy with a thin layer.<<

Interesting. Smaller sizes, as in many crafts, make it both harder and more necessary to make fine adjustments. A larger, or at least taller, container would give more margin for error. Not to mention more headroom for putting in sticks and things, which ended up being much of what I most wanted to have in there.

Still, for a first try, they're quite cute. And now I've got some confidence to start thinking about a larger one and keeping an eye out for suitable containers. Win!