Alright, here we go. Here are all my components assembled: concrete mixing tub, wire mesh, soil heating coils, and zip ties.
I drilled a bunch of holes in the mixing tub to allow drainage.
Then, I loosely zip tied the heating cable to the metal mesh. The heating element is 12' long and is supposed to warm 3 square feet. The mixing tub is 2 feet x 1.5 feet, so perfect size.
I then added a 1.5" layer of pumice to the tub. It is lighter and will still retain moisture. I laid the mesh and heating element in place.
I then added another 2.5" of pumice in place.
After about an hour, the air temperature was 50 degrees and the surface temperature was 55 degrees. About 1" above the heating element it was 65 degrees so I removed some pumice and nestled my boxed tree. I filled up the sides with more pumice so it is sitting about an inch deep.
The tree is a coastal redwood that i just repotted. It was in heavy clay soil and got beat up a bit during transit so it was relatively weak. During repotting, it didn't have a very large root mass so I really want to give it as much help as possible.
The nice thing about this setup is that it is modular and not too heavy. If the air temperature gets down to just about freezing, I can leave this tree outside. If it gets into the 20s, I can just slide it into the shed where it never gets below freezing.