Filling the hole won’t help that much, because you’re limited by the adhesion between the edge of the hole and the filling.
Skinning the area with some fiberglass or carbon fiber would be an option, as would just epoxying on some reinforcement plates (either composite or metal).
Just beginning to assemble, so the deck is bare essentially. I couldn’t find a good top mount deck with the right shape, size, and enclosure available.
I’d get some open cell foam to fill the void and glass over it on both sides. Just remember when you reinforce an area the stress is often transferred elsewhere. I’m not an expert but I’d try to keep the reinforcements even across the deck.
@glyphiks did a repair to end of a demonseed? Which had been hacked. with a fibreglass sandwich too and bottom of deck. Have a search for that, I reckon.
I wonder if there’s any benefit to filling the void with something solid - like route/jigsaw/sand a chunk of 2x4 to fit tight and seal it to the rest of the deck with wood glue. Then fiberglass over.
The wood chunk wouldn’t do much for truck support, but it may act to thicken the tail section so it’s less likely to snap
If it’s a decent new deck, why would it snap?
Shouldn’t be any extra load by using top mount vs drop through.
Stiffening it cos you want extra strength is another matter isn’t it?
the plug isn’t likely to help, apart from as a base to lay fabric or another veneer over imho.