Well hell, as luck would have it I needed to enlarge the opening for the sanding drum and add a thinner shim. Here it is all set up to start routing 1/16" from one side. I did the same to the other side.

When I made my first trial run with my sander I found that my hook & loop did not "expand" as much as has been stated by others. Perhaps it was because I used industrial grade materials, including Mirka Autonet sandpaper - a mesh sanding medium that avoids clogging and runs cooler. So, I had to use thinner shims under the top and enlarge the hole for the drum - a bit of a PITA, but not that bad, all things considered.