You should know as well as anyone that we NEVER have the largest aperture scope we want! ;)
PLEASE! PLEASE! PLEASE! Do NOT make your roll-off walls so low that you only barely clear the scope. Allow for the largest diameter scope that you ever plan to own, and then allow a bit of cushion. Unless you are on the highest point on a small desert island, you won't be imaging right at the horizon, so slightly taller walls won't hurt you.