Here, I am using a piece of .001 dental matrix strip (ask your dentist for a few) to check around the edges of the joint and…. “Oh crud! How did that happen?”

You could also use a feeler gauge for this. (BTW - a sheet of paper is about .004.)
And here’s how I fixed it. I placed some CA glue on top of the strip and ran it back and forth through the joint. Then I clamped it.

It worked! And it probably saved this build from having a weird buzz after completion.