First, the pour. Don't need much ultracal for this, which is good because I'm almost out. I'm always bad at judging how much I need, yet thankfully it's usually too little, not too much. Didn't have that problem this time though.
Upon investigation, I didn't take a picture of that step, but you can imagine it.
Next I place in the fake plants. I got some aquarium decorations and some from a craft store, so I arrange these in an appealing manner. I mixed the plaster too runny so the plants couldn't stand up when I put them in, so I just had to wait a few minutes.

Next, I placed the larger stones. These give some shape to the ground and things to climb on. I also put in a flat rock to act as a feeding tray.

Then I sprinkle some crushed gravel over the plaster, to give the ground some texture. Aquarium gravel would probably work for this, but I used some stuff I got from the craft store. Don't add too much, this is just to give a bit of variety to the ground.
(I almost forgot to take a pic at this step, do I kinda started the next one already)

Then, sand. I pour sand over everything, completely covering any remaining plaster, getting it between the plants and up to the edges. This one you can be liberal with, as you'll dump out the extra later, and it helps fill in the cracks.

Then, just wait for it to cure.

Once it's fully set, dump out all the loose sand, and brush away any that's not firmly attached. Don't have to get everything out, but it keeps it nice.

All that's left is to cut vents in the top and cover with mesh, and add entrances in the sides. This box has an airtight lid so it should be hard to escape from, but I'll also put on some fluon to prevent escapees while feeding.
I may have been a bit overenthusiastic with the decorations in this one though... It's pretty crowded in there, and might make cleaning difficult. Let me tell you, those little plants look a lot smaller before you put them into the box... I might make another that's a bit more open.