The thicker d-cell is to help support the box at the bottom.
Putting the box in first is an interesting idea and should work OK.

On a new board I leave the box full size, put the 80kg/m3 foam in then router/cut all the way through the deck, then cut the box level with the deck.
Seal the hole in the deck with plastic and tape.

round the bottom edges of the hole, glass/carbon the bottom.
Wrap the box in 2 layers of 150gm glass with enough to overlap into the bottom about 2cm to 3cm, I rebate the bottom to accept this extra glass before glassing.
wet out the hole, wet out the cloth and pour some resin into the hole, lay cloth over hole and insert box.
weigh down a bit of mono film over the glass overlap and box.

I then glass/carbon over the deck, this really ties the box to the deck, but in your repair situation would entail more cosmetic problems, and you also have the existing plugs to consider.

That's a massive hole you have in your board.

Boards finished . Ankles good enough to test . The end result is im happy with the job i did has to be a million times stronger than the original. Its was an expensive repair that this board can only just justifier. Still got some spare material so it will work out cheaper in the long run . Makes me want to strip another old board & reposition mast track & footstraps . I took a lot of pictures that i can post of repair if anyboady wants them otherwise i couldnt be bothered .

Pls post pics, I'd love to see how it turned out

actiomax said...

Ok got the fin box

great to see your happy with the job action max, i have noticed the top of the box doesn't go through to the deck, as a precaution i would sleeve it so there's no join under the deck, as a matter of interest i have sleeve all my box's so i can use smaller screw heads and less damage to the box if you hit something and pull the screw through

10 points mate, very nice job!