While there is some space there, on my aircraft I am installing a small box back there hidden by the canvas to put my compressed air bottle for a reserved start on the M14. Without the box, I would fear the result of all those Aeronca pilots we've read about who found out the hard way what a lose seat belt can do
I figured the thing had to be somewhere it could be easily wound up and turned on and off by the ground crew. You have to pull the guts out of it to tear off the slip of graph paper, so it would yanked quite a bit, as there is only enough paper for a single 2 hr flight or so.