Bonsai Nut
Nuttier than your average Nut
For the record, I can't stand cactus mix because when it gets too dry, it repels water.
Does the same for me too if I let it get bone dry (like on my cacti). If I mix it with more pumice it helps. I certainly don't use it for any finished bonsai but it seems to work well for the junis that I have pulled from the field. They are in shade for quite a while and I never let the surface of the soil completely dry out at least for the first 6 months or so.
Quick comment on the plastic tent idea: something else that I have done when I need to transplant junipers in the summer (at the WRONG time of year for this in Southern California) is to wrap the entire tree in burlap, including the entire foilage ball. The burlap provides shade and holds moisture (especially if you water the burlap 2x a day for while) without baking the tree or cutting airflow. I have done this with large adult landscape junipers - digging them up and moving them in August in full sun - and have never lost a tree.