Class C cabover lights replacement problem

I'm repairing water damage to the cabover on my 2000 Coachmen Santara Class C. Water entered through the cabover lights. Behind the lights was a open area about an inch deep that was filled with "pink" insulation. I removed the lights and used a grabber tool to pull out the wet insulation. Everything is dry now. My problem is, the luane that was behind the filon came apart when it dried. Now I want to reattach new cabover lights but there is nothing for the screws that should hold them in to "catch" to. I can't figure out how to get anything in that area for a screw backing. The holes for the lights are about 1 inch in diameter, and each had a screw about one inch to each side of the light hole. A piece of wood small enough to go through the hole is too small to catch a screw without breaking. I have no access to the back side without pulling down the ceiling. Today I filled the cavity with spray in foam insulation. It's not sturdy enough to hold a screw.
Looking for ideas!
Thanks,
Paul