The black area us the FCS single box, a new unused one has a 2mm wall around the top surface, remove the old box, try to save as much foam core as possible, make a few vertical tubes using a screwdriver , theses are to allow the resin to flow downwards and attach to the inside of the deck, mix up resin, add some filler powder to the mix , to thicken it out, but it's not really necessary.
lightly sand around the "plug hole" surface, and then leaving an inch or two of surface exposed, mask in a circle around the "plug hole".
Pour resin into the plug hole and down into the tubes, keep pouring until the resin is about 15 to 20mm from the top of the hole.
Insert the fin box, and check a few things,.... the fin slot is in the right orientation ! the top surface of the fin box is flush with the surface of the board, and the surface is level, ( the wall around the new box will be 2mm above the surface, and holding out the resin that should have overflowed out of the plug hole, having been displaced by the fin box,
This excess resin will flow onto the sanded, and exposed area around the plug hole,
Check everything is level, and allow to harden, you may find that some bubbles in the resin have risen to the surface and caused tiny blow holes, fill them later !
When fully hardened, using an orbital sander, or block, sand down the new resin until flush with the original surface, this will also sand down that plastic wall.
In the rough sketch, please note that the foam has been undercut beneath the original fibreglass surface, this will give a stronger and larger bonding area of the resin/surface bond.