Fourstroke,
Things like having the clunk fall off, etc., happen to all of us. Don't be disillusioned. We all go through it. At least you found the problem, found out why it did what it did, and you fixed it. It'll never happen on your next plane.

Another thing about the cowl... is there a lot of 'dead air' space in there? Sometimes that can be filled with balsa or foam so that the air gets channeled around the engine and not into pockets inside the cowl. You just have to re-check your CG after you do any work on the nose.

I'm sure, in the end, the problem will turn out to have a very simple solution...

Bob