Fin box too big for fin

Got an old single fin with the described problem. Any good solutions for filling the gap?

Cut some shims from a plastic milk bottle.      Or a light coat of sanding resin both sides of the base.       Let it cure hard before fitting.      If still loose, sand lightly, and repeat as necessary.

How about such tape around the base. As temporary

Bill’s suggesting is an actual fix.   Alternately, you can also do a short term hack:   some people have wrapped the base with sand paper (obviously, for wet sanding) to add thickness and grip.