If you truely want to knock the rat out without much expense or wanting to be extra safe for the snake, what I do personally is go out and buy a can of dust Off ( Compressed air for cleaning). I stick the rat into a plastic bag, stick the Dust Off straw into the bag and fill it up until the rat passes out and then they die which happens normally withen 15 seconds. Much more Humane than any other way ive seen ( Although the Plastic bag could be improved if you wish to something else air tight like a plastic tub with a hole poked in it for the Air straw).

I feed live personally but thats how I took care of my extra rats to freeze after.