And why support the feet at all if the goal is slow asphyxiation?
Here is a link on crucifixion: https://en.m.wikipedia.org/wiki/Crucifixion
Here is what I propose: a T, which is simpler and easier carpentry, reusable as they point out, and attested to historically. You tie the mans wrists to the crossbeam, haul it up only enough to get his feet about a foot off the ground–two guys with stools could do this–THEN nails his hands down, just to be sure he doesn’t wiggle free. Then you lash his feet to the pole, so he can’t move around, but so they give no support to his upper body
His diaphragm is forced to work much harder. Most of us, we get some lung motion by raising our shoulders. This is no longer possible. The ability to inhale deeply is compromised, and breaths become shallower and shallower. The death is like a slow drowning.
In Christs case, though, there was such a ruckus that he was pulled down prematurely by a Roman and killed with a spear, just to put an end to the circus.