Something free draining, but holds a decent amount of water and nutrients. Colanders dry out faster than containers without holes
This and what
@Leo in N E Illinois said.
Main benefit of a colander is that it dries faster, which is also the main downside. If you are not keeping up with the watering your tree will suffer. If you are not watering consistently as it dries you will not reap out the benefits. From Jonas and other artists that use colanders, you need to water and fertilize more frequently when using colanders, pond baskets, etc...
Eric S. did an experiment with colander vs terracotta, but in my opinion, he failed to follow Jonas advise for colanders. He kept watering and fertilization equal, negating the benefits of the colander all together.
phutu.com
If you look at the growth that Jonas get from a 6-8 yrs old JBP, it is comparable with the growth obtained by ground growing, but much more compact. In the case of a juniper, the same holds true.
With the higher price of akadama, and with the fact that many people advise to only use it on trees in refinement I would use pumice/lava and either Bonsai Block or Monto Clay from Bonsai Jack, or any other substrate that holds water in the same manner. Monto Clay is the same as turface, but Bonsai Jack selects particles that are closer to 1/4" vs the fines that come in the turface box. And, I don't know how or why, they are slightly rounder as well vs the plate-like MVP and other turface.
Another option that Telperion used was the Rootpouch, that root prune by entrapment vs air pruning. If you use the above ground bags they will dry out slightly slower than the colander, but faster than a pot.