It does not have to be metal.
It should retain its shape if it gets warm and preferably fire resistant.
It might help cooling being a conductor.
I have seen a number that have a plastic infill with a bolt through that, cooling here is obviously not an issue.
Yes, in the absence of the DIYer getting hold of potting compound, you might be able to fill the centre with epoxy. You would need a pillar drill though and possibly a milling attachment unless you can be sure your top surface will be true.