Make it adjustable if you intend to have more then one person use it. Two pieces of telescoping pipe, a series of holes, a pin and you are most the way there.
Height, torso length, shooting style and age all factor in.
I bring a 5 gallon buck to the range all the time to sit on/carry stuff in as their stools are all to high for me and I have to crouch over so much that my back soon begins to hurt and shooting goes to hell.