The channel was about 1" or so wide and a little deeper along the length. There was a lot of grind, test, flex, cut, test, flex, etc.
Thick aluminum wire was laid along the length, folded, laid along the length 3 or 4 times until the groove was filled. Copper work hardens and would have been too difficult to bend and twist and flex the way we needed. It was there to support the trunk as we made the bend. Size 6 copper wire was wrapped around the trunk, twice, to support the bend and keep the tree in position once the bend was complete.
The prep work took the bulk of the time, 2-3 hours. After a break for dinner we returned and did the major bending in 30-45 minutes. It took about 3-4 different guy wires to get the full bend, place one then remove one, move the jack, bend some more. In the end only one guy wire, and the main copper wires and internal aluminum remained.
I'm sure there are nuances that I'm not remembering, but that's about the gist of it.
With collected junipers they seem to become more flexible in new soil and fertilizer after a few years. Lots of new growth on this one means that it should recover quickly.