The whole idea of lethal injection is stupid, anyway. We have a simpler means--anoxic environment. Painless, messless, leaves an intact corpse and no fiddly things to go wrong.
Just pump the execution chamber full of nitrogen. It's not toxic so when you're done you can just vent it outside.
Um, suffocation may be painless in the strictly physical sense, but it is far from it in the practical sense.
There are some gasses (CO is one IIRC) which fuck with your brain enough that you don't realize you're suffocating... those might be an option.
However, running a gas-chamber isn't actually that easy. There are quite a few things which can go wrong, most obviously not getting the concentration high enough for long enough. Making a "quick suffocation" environment is more complicated that just sealing a room and pumping in another gas... the air in the room has to go somewhere (or somehow be rapidly depleted of of O2). That is why adding highly toxic gasses/vapors to the air in a sealed gas chamber is the 'traditional' method. (BTW: Gas chamber has a lot in common with lethal injection actually... and a lot of the same problems.)
Pretty much every execution method (at least if the goal is to minimize cruelty) other than massive trauma actually gets complicated when you get into the details.
ETA: I'm opposed to the death penalty... very little or no upside and quite a bit of downside, but here is my proposal for a fool-proof method if you really want it.
The goal should be to render the brain incapable of thought quicker than the condemned could perceive the execution happening. It couldn't be any more painless.
The most reliable way of doing that is to physically destroy the brain very quickly. And the most reliable way of doing that is to simply smash it into goo with a high-velocity impact. Bullets/bolts (or lots of other contrivances) could do that, but they are more complicated and less reliable than ideal...
The execution device is simply a heavy weight (a 3'x3' steel block is good) on a long cable and an immovable object (a 'target' attached to a reinforced wall). The pivot/attachment of the pendulum is directly above the execution chair, and the chair faces perpendicular to the way the pendulum swings, since there no reason the condemned has to watch the pendulum coming at them and heads squish slightly more easily from the side.
The pendulum is hoisted and held.
The condemned is strapped into a chair with one side of their head resting against the target. The chair includes a blinder on the opposite side (the one facing the pendulum). A hood or blindfold may be provided, or refused. I'd also have the condemned facing the viewers.
Any secondary hoist/hold cables on the pendulum are detached so that it is being held with a single cable which has a release... perhaps an explosive bolt to be really fool-proof.
If something like an explosive bolt is used as the release, the mechanism to actually trigger it can be as complicated as you want. You could have something like the classic firing squad with multiple buttons, only some of which actually trigger it, or single simple button (which I'd prefer the governor to have to come down and trigger from the viewing room looking at the condemned.)
Post execution, you'd need a body bag, a hose, and a bunch of bleach.
PS: An even more humane method (and simpler in a way) is a hat made of high-velocity explosive. But I can't figure out how to do that indoors. If we ever go back to public executions though...