Isn’t there a machine which kills beef cattle quickly and efficiently?
Why not use some variation of that?
I am more of a repent by suicide type of guy. For the rest of the convicts life, he will live alone in a windowless room where sticking out of the wall is a button and right above that a hole with where the bullet will enter the brain. The ony way the bullet will fire is the convict must stand on a floor plate, place his eye on a retina scan and once positioned activates the device with finger print.
It is only at that point where the convict can push the button and the bullet will enter the brain, killing the convict. the body is removed, the cell is self cleaning and is ready for the next convict.