Are the wheels supported on the tailgate? If they arent putting weight on the tailgate when it's down, you could simply remove the tailgate. If they are resting on the tailgate, a piece of 3/4 inch plywood, cut so that it only projects out the rear of the bed far enough to support the ATV. With the front wheels also being on the plywood would have plenty of counterweight to keep it from tipping. And you would just remove the tailgate altogether.
You would have to secure the plywood for when you are loading and unloading, and ensure you have the atv strapped down well.