Those may simply be gelcoat cracks and not a factor of insufficient core strength, rather inferior gelcoat. If you're certain you need a better backer, I'd use G10 and take up the irregular space between it and the underside of the GRP with heavily thickened epoxy. Getting it perfectly flush with the outside requires a bit of creativity. I'm thinking you drill the mounting holes in the G10 and hand tighten some mounting hardware, while it all sets. Getting those hand tightened bolts back out might be tricky.