Make or buy a donut or short tube that can orient to the face of the rack's "braze- on" and provide a hard face for your adapter.
OR
Use a file to remove the two side rails from the adapter, then file from the top until it squares up between the rack's mount and back rail.
If it were mine, I'd do the second because it'd be more rigid, but I'm good with a file. Otherwise the first is probably fine.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.