when I drill out the columns in the board and fill it with epoxy for the t nuts, should I drill the column wider than the head of the t nut (so the prongs of the t nut grab epoxy when I insert them) or narrower than the head (but wider than the barrel) so the prongs grab wood? I would assume the former, so there is less potential for water entry into the board but I’m not sure.
Suppose the widest part or "head" of the T nut base is 20mm.
I would drill 25mm holes all the way through the board before glassing.
Next fill them with a strong epoxy/fibers putty (microfibers, wood flour, milled glass, etc.).
Drill out so you can fit the barrel of the Tee nut in the holes, maybe 1mm oversize so it slides in easily.
If the Tee nuts have prongs, flatten then with a hammer or pliers. They aren't needed (if possible, just get flat weld-on type tee nuts).
Get some long screws to fit the tee nuts, and wrap them in masking tape or similar leaving only enough threads bare at the tips to engage the depth of the tee nut.
Don't get petroleum jelly on the barrel or head of the tee nuts.
Insert tee nuts into holes from deck side. Secure to deck with tape, being careful to get a good seal.
Turn board upside down making sure the tee nuts are supported from below.
Coat the screws with petroleum jelly, and screw them into the tee nuts.
Pour epoxy into the holes around the screws from the bottom of the board.
When cured, unscrew the screws.
You now have tee nuts secured into strong epoxy holes, with enough clearance to insert screws easily.
Plug holes with surf wax, plasticine or whatever to protect from resin, and glass the board.
After glassing, open the holes with a drill -carefully.
If you feel confident, you could try potting the tee nuts and screws directly into the 25mm holes in one go.
If you feel too cocky to get a good result, you could try doing any of this stuff without using the foil plate or an accurate template to place the screws in the holes.