Really enjoying this project Norman, nice casting work. too!
I have a concern about concrete filling -- which may or may not be valid. Up to those with experience combining these two materials to say.
Portland cement is highly caustic and aluminum is particularly subject to attack by alkalis. Will this cause problems, not only during the filling stage, but also down the road? I think even cured concrete remains basic for quite a long time. With moisture from condensation (aluminum a great conductor, concrete much less so, and porous) I just wonder if problems like corrosion would cause either separation, or the opposite, expansion, or both in different places?
Could be I'm wrong -- hope so -- concrete would be a nice inexpensive vibration damping mass if not.
ps, I remember reading in an old ME about a large lathe built after the War out of concrete as the main material. And here in Vermont, at the Precision Museum in Windsor, there is a late constructed of marble (quarried here).