Once upon a time when I was on container ships we wondered if US customs and their reputed combination on ignorance and arrogance* would have them walk into a controlled atmosphere 40" container to inspect it.
They would have needed an angle grinder for the safety interlock, and a boxcutter for the gas curtain, but would then have found themselfes in an 5% o2 and 2% co2 mix.
*Somewhat extrapolated from experience with coastguard port state inspectors.
Dialling the settings down to 0 could get you a mobile gas chamber using existing and easy to use technology. (It is based on an compressor and a filter fine enough to separate n2 and o2.)
Your prisoner could be escorted/dragged into the container by guards wearing emergency escape re-breathers, another off the shelf component.
It is not like there is a shortage on options.
I can't see fitting a prisoner easily into a 40" container. Do they make them any larger?