I have been looking through a LOT of terrarium threads here and I don't see any mention of plastic canvas. It is so useful and can be made into any shape in almost any size. The holes allow it to hold silicone and many other bonding materials. It gives places to bind on twigs, sticks, rocks and a lot of other things.

I have made everything from tubes to cones, boxes, squares, prisms, rocks and many others things I can give a name to. It comes in pre made shapes for making shapes easier.

I haven't done this but I bet that you could easily coat it with titebond or even concrete and it would hold the shape very well and allow for very thin coating for lighter weight objects and structures. If you don't want the canvas in there after, line it with wax paper or use plain brown paper so that the paper can be washed away if wax paper mix get bonded too tight.

Make a tube: Take 2 cans different sizes but close together, cut off both ends, line cans with wax paper, put some plastic canvas inside, spray poly foam inside on plastic mesh for an easy light weight tube. Easy mushrooms made with the pre-made domes using titebond, poly foam or whatever you like to use.

If I knew more about the materials used in terrariums I would make some for myself but I have a lot to learn when it comes to that. Titebond, poly foam and all the rest are still Greek to me. I have used poly foam but only to insulate walls and would likely have a tank full of it if I tried to use it for a terrarium.

I have used plastic canvas for molding clay and ceramics with great success. It was so easy to make a mushroom cap come out perfect. Making clay tubes was just as easy. Folding wet ceramic slip that hasn't dried too muchover plastic canvas allowed it to dry really well and gave me some texture in the ceramic for trying to get aquatic mosses to grab onto it. Remember that ceramic needs to be fired in a kiln and so do some clays, some clays need to be baked but I am not sure if baked types are safe or not for terrariums.

If I can find any of them I made I will post pictures. Most were sold in my parents ceramic shop when they had it and the rest, let's just say 3 young children had fun with them.