Yogi...... yes...." make a foam plug that is to the hull profile and bonded into place then fibre glass over it"
Balsa would be fine....I just thought that foam would be easier to contour.....
The suggestion from boatmadman ....of the false hatch is well worthy of consideration.....that way you could mechanically remove the foam after the resin bonded cloth has cured & then add a little resin reinforcement internally
From the images shown the foam plug would need to be ~~~~~140 mm long ....x ~~~~90 wide.....x 80 high & the profile of a triangular brick
with a rounded top
........so am guessing the foam + the glass cloth + the liquid resin & hardener would not exceed 150 gms ??????????????????? & as you note you may need to redistribute or reduce ballast to compensate........Derek