Step 1: Create hardwood shim to fit iPad corner
I made a shim out of oak wood, about half the size of a fingernail, flat on one side and concave curved on the other, that fits next to the iPad camera and makes a flat surface all the way around the lens.
I used a fine-point marker to mark the position of the shim on the iPad case, then used a drop of super glue to glue it to the case. If you ever change your mind about having it there, it can be removed with careful scraping without damaging the case.
The Photojojp lens ring sticks to that solidly, and the problem is solved.