Hood swiggler, since your fin is swiggling too much, you are definitely doing something wrong.
The usual way of putting these fins in is to put the back part with the metal tabs in first. There is usually a gap in the plastic sides of the finbox into which you can slide the tabs. the plate for the locking bolt at the front of the fin also slides into the horizontal grooves. That gap must be there as you got the plate in.
Sometimes in manufacture the box or the fin are not quite standard in dimension and a little sanding ( best on the fin ) is required.