However, the curve around the bottom is perfectly sized to 77mm. So I also purchased an empty 72mm to 77mm step-up ring that will screw onto my lens.
Then, I trimmed the excess cardboard from the filter card so it fit perfectly into the ring.
It held pretty securely on its own, but I secured it with a judicious application of gaffer tape. Now I have a “permanent” solar filter that won’t fall off.