The flat sanding attachment needs a backing plate, so I’m using an elaborate series of clamps to install it in the proper location.
One of the challenges is that the platen needs to move with the pulleys — the adjustable part of the attachment — and not the support arm. The layout is snug but it works. There’s also room for a graphite pad, like you’d find in a belt sander.
For all my planning, I still managed to screw it up. There is too much displacement of the belt, so I’ll have to cut down the height of the platen supports by making some dado cuts.