Foaming the other cavities was a bit easier.

For the other cavities, I just drilled some 1" holes and poured the foam in. Here you can see some of the holes after the foam installation. I just samded down the foam inside the holes with the dremel down about 1/2". This is being filled with thickened epoxy with a bit of chopped glass. I beveled the edges to give the epoxy a bit more surface area to bond to.