State legislatures in states with capital punishment need to have an alternative means on the books of the firing squad.
It is inexpensive, readily available, requires no special expertise, is very effective, and can be used as a ‘default’ in case their preferred means of execution is blocked.
It is also very hard to challenge on appeal.
Or you could use automated machine guns, like in Thailand.
It could be kind of messy to clean up afterwards, though.
https://www.youtube.com/watch?v=-c3WIL4JSAU