Gato,
Maybe you allready have answers for this but I'm curious about the foam filling issue, same as Rolf:

from my experience working with foam, poured into tight, confined sealed spaces - it will probably deform the mast shape, unless you provide some "exit holes" for excess foam to get out. It will definitely add rigidity to the mast, but may also lead to damage.

This is much nicer than the F16 forum. Good luck with your projects.