They really need more shallow and long tanks but yeah, for now, that turtle will love that hundred gallon. I would make an above tank basking area for that tank. where you just make a piece that sits on top of the tank, about half as long as the tank. with a ramp into the water. I think it would be better than stacking rocks that high. I think that would take up to much swimming room in a tank that tall. Mine is 6 foot long and only 18 or 20 inches high so yours must be waaaay taller. Still that is a way bigger tank than most turtle owners keep a turtle your's size. He's going to love it.

try one of these, there really simple to make out of stuff you have laying around