Very nice! Did you use any reinforcement (like wire mesh)? Have you chosen a tree species for your planter yet?
I choose deep orange bougainvillea from my friend which easy to care and of course wire mesh are necessary to hold the form.
Hmm concrete, and here I am messing around with mycellium to make decomposing pots for transplanting.
very interesting Bonsai container, I like it! I can just see a juniper trained to weep over the edge and down towards the ground from it's perch on a nice pedestal.