turn a bucket upside down, cut a hole in the top, drop mouse in. snake goes in, can't get back out.
That probably works for most snakes, but garters don't eat mice. You might want to use half a fish (if it's cut in half it will smell more) and put the hole high on the side of the bucket (the snake might not be able to get up to a hole in the top. Good luck. ~TR~