You should be able to get the mast box from Fiberglass Supply or direct from Chinook
You would need to rout a wider slot in the deck of your board, maybe add some hard foam or wood strips in the bottom and sides before you laminate in the mast box.
You need to spread the load out over a larger area.
Then fair in and glass the mast box into the board with the correct type of resin (polyester vs epoxy here).
Should not be too difficult to get a good looking install.
Hope this helps,