Aloha,
I created this little bonsai a few years back. It measures about 3" tall. The pot I made myself using a very small plastic tupperware bottom in which I poured a shallow layer of melted wax. After the wax hardened, I carved out a negative space where I wanted the feet of the pot to be. I reinserted the wax into the mold (tupperware) and poured in my cement/cinder mixture, shaping the inside of the pot by hand as it started to firm up and draining off any excess water during the process. Next, I transplanted the tree into the pot on top of a layer of coarse cinder, which was mounded on top of a small plastic grid to prevent the drain from clogging. To finish up, I laid down a layer of moss around the tree, which I peeled off some rocks out back.
Photo and editing done by me.