I said earlier that using a mask invites a lot of things going wrong. Including, but not limited to, insufficient gas flow to the mask so it ends up being a de facto obstructive suffocation, or the mask allowing re-inhalation of exhaled breath which would both draw out the process (since exhaled breath still has quite a bit of oxygen in it) and allow CO2 to build up. I wonder if either of those possibilities actually contributed to what was observed and reported.
A chamber has unfortunate associations and guarantees a lot of extra preparation and fuss, but a clear hood something like what used to be called an "oxygen tent" in hospitals, flooded with a high flow of nitrogen, should be sufficient. Let the executioner wear an oxygen mask of his own, if he's afraid of "escaped nitrogen."
Except that the convict knows their time is nigh and may well panic.
What I have in mind is pretty much indistinguishable from a typical jail cell, and no restraints are necessary.
Grim. That would be absolutely terrifying for me.