Our contractor recommended running 2x4 horizontal on the outside of the pole. Just matched up to outside wall 2x4s. Then staple r19 to the 2x4s
That's pretty much what I did, but I also added a vertical 2x4 in the middle between each pole. Without the center 2x4 the middle two lengths of R19 don't have anything to staple to.
This will give you an idea (south wall):
Then I got a bright idea and realized the area between the 2x4s was free space, so I added 1.5" foam insulation between them on the south and west walls that get the most sun (wasn't quite finished with the foam board when I took this pic of the west wall):
I haven't added the mini-split AC or the radiant heat system (PEX is in place) but I can tell you that if it's a really hot day and I go in there for the first time at noon, it's quite cool inside. All the walls got steel liner panel over the insulation, the ceiling has steel liner panel, and then fiberglass insulation over the top. I will eventually had another 10" or so of cellulose insulation over the top.
This is after finishing all the insulation, and hanging the steel: