Update: We are just back from 560 miles/round trip without any vibration of the front storage box. Before the trip, we discovered a bad rivet on the curb side end of the box lid. The only way to fix it involved removing the box from the trailer tongue, which we did. Two things we noticed in that process: three of the four attachment points were shimmed with washers, and on the bottom of the box was written "19." (Do they use the same box on a 21 as on a 19? Does that "19" have nothing to do with the size of the trailer/box?) Anyway, the nuts/bolts on the attachment points seemed to be snug - the box wasn't loose on its mounting. So, Skip put in a new rivet and reinstalled the box.
Loading things for the trip, we decided to (finally) go with Jon's suggestion (see post #22), putting a towel on top of the center brace, to give a little extra pressure on the center point of the lid. Skip also tightened the staple on the street side hasp another turn, for it seemed to have loosened a little again.
With several variables from our last time out we can't be sure what fixed it. (It also may not be a permanent fix... but I'm hopeful!) Maybe in reinstalling the box, there is some slight difference in position? It would be easy to check to see if it's the extra pressure from underneath the center of the lid (by removing the towel... but I'm not gonna!) Or maybe we finally got that street side hasp the proper amount of snug (along with the center brace and hinge rivet no longer being broken)... Maybe it's a combo of all of the above.
We're pretty confident it wasn't just that we were driving on a calm day, as today was a bit blustery (as we came through a rainstorm).
So that's it for now, folks. Hopefully, that's the LAST of it!!