Don't use cactus soil because it's generally 99% peat moss and very bad at either not draining at all when wet, or else not absorbing water when it's dry. Use very small gravel from an aquarium store, mix it with a little (30%) potting soil and see if you can break down the wooden pieces in a bag of orchid mix because the pieces are often quite large... 1/8" size is as big as you want and 1/16" would be better. They are relatively soft in those bags and some will break apart with your hands. Do NOT use gravel on the bottom, but use plastic canvas from a craft dept. to cover the hopefully decent size hole. It would be far better to let the tree live outside for life so you wouldn't be faced with when or how much to water, but you would still need to protect it from heavy rains, so as watering - not often at all, but well when you do it - will be your biggest challenge. Full sun of course.