I think that the " the cells were 6-12mm in diameter from the photo" is a little off. I think that the normal cell size is between 3-5 mm. For a quick how to go to.
http://www.fibreglast.com/contentpages-Fibreglass+Repair+Composite+Repair-123.html

In the past I have routed out the bad core section and then vac. bag in a new core, then bag on the skin. If the inner skin is fractured this must be repaired first!