Air tight room. Pump it full of of pure nitrogen.
The air we breath is 78% nitrogen. We have plenty and requires no licensing and very minimal training.
No pain. No struggle. (Nothing more than a petulant child holding his breath.) Nite nite forever. Works every time it has been tried.
If you don’t tell the convict when the gas flow begins he won’t even hold his breath. He will just lose concisiousness and pass away peacefully.
Or as peacefully as someone who knows he is there to die can. After all he knows that he will soon meet his maker and be judged fo his sins.