Hi Steve C,
What is the problem with your favorite weed fin in the Serenity?
If it goes in too far, then you may be able to glue in a couple of
shims on the ends to make it fit correctly.
If it stands proud of the bottom of the board, you can always take
a fairly large diameter rat tail file and work the ends (the big lump is
normally found in the back and fairly near the bottom of the board
(vice further down in the fin box) so work on the back first) of the fin box
until the fin fits at the correct depth.
I know this is backward, but in this case you want the fin to fit your other
board first, so correct the fin box in the Serenity to make it the same as
your other board.
If the fin is too tight on the sides, a little work with a double cut flat file, inside
the fin box, and you can fit you fin nicely. Use some soap as a "marking media"
and file the shiny spots down in the fin box in the same manner that you would
the fin if you were going to change the fin root.
If your fave fin is too loose on the sides of the Serenity fin box, you could fairly
easily add some thin alumium shims to the sides.
Cut the shims from either a soda can or maybe from a plastic milk jug.
Put a little thickened epoxy on the back side and insert the fin with some
Saran Wrap around the root to act as a release sheet.
A little work with a file, and you'll have a perfectly fitted fin in both boards.
Hope this helps,