I would put a heavy plastic over the sand and start pouring to the height you want.
I know you did not ask about plants but......I would not worry about the plants after you get it set up. There are a lot of plants that grow slow and because it is small you are not looking for a lot of plants. A few small ferns and a Cryptanthus or air plants they would never get big.
I have had this in the little bird house over 3 yrs, It is just now as big as my hand most of that time it was the size of the palm of my hands and made lots of babies. You can use them in your other projects.
