OMG Yuppy how many board repair threads are there here?


that say don't use polyester resin (or don't listen to bloody Bunnings employees who say surfboard repair kit = windsurf = SUP etc)

All is not lost though.
You can build up that big void with any HD polystyrene or normal surfboard core foam. Especially as the bottom laminate is intact and gives u a guide.
Cut it to fit as close as possible, butter it well with epoxy resin and q-cell mix and fit it. Then shape it with a sanding block and glass over (4 oz x 3 layers with epoxy will be kewl).