The idea is to let it rot off. It's only there as a temporary back up plate to facilitate the repair. That way everything can be done from the front side. But G10 should work fine, too. G10 is circuit board fiberglass, for those unfamiliar. It is readily available in many different thicknesses. I get G10 from MSC supply.

I've only done maybe 20 repairs to sandwich core boats, myself. I have done hundreds of radomes, floors, flaps and ailerons, though, with every type of construction from metal over balsa, glass/carbon/kevlar over nomex h'comb or various foams to metal skins over metal honeycomb.

Jimbo