How about extending the top connector with a steel or aluminum flat bar, with drilled holes? There's two holes in the existing top connector, so drill two holes for those, and one more for the fork mount.

There would be quite a lot of side load with loaded panniers, with the bottom securely fastened and the top on a bar extension. Would this affect the adapter or allow the load to move side to side a little? It might. But it's quite cheap and easy to try on a test ride.

Lowes carries these. I can't link the page easily, but this google search works: The Hillman Group 3-ft x 1-in Plated Steel The 3 foot x 1 inch bar is 1/8 inch thick.

Would the extra 1/8 inch thickness affect the rack at all? I'm guessing it'll work. Or you could put the steel bar on the outside of the rack's mount--that sounds better.

Perhaps the aluminum 1 inch wide x 1/8 thick would be stiff enough. It partly depends on how long it needs to be. Aluminum is easier to drill and saw, of course.