You can adjust the opening if needed to get a good fit. I would try sanding it. Just go easy, taking off a little then checking the fit. You don't want to remove too much and and up with a sloppy fit. Don't alter the thickness of the vertical stabilizer base itself. This could weaken the piece and cause a failure in flight.

It's also possible that there could be a little excess glue in the opening, especially in the corners. That could prevent a good fit.

A picture would really be good. somebody could probably tell you exactly what to do based on that.