I like the idea of a pre-formed sleeve inside the brass. So I changed my plan somewhat.
I part filled cleaned and deprimed 30.06 cases with builders glue, the stuff you fix drywall on with. It doesn't shrink when it cures and is strong without being too flexible or brittle. Then put 2 layers of aluminium foil round a 6.5mm rod and pushed it into the part filled case, untill the glue came out of the primer hole, then this was blocked and the rod pushed in farther untill the rest of the case was filled solid with glue. I pulled out the 6.5mm rod leaving an aluminium tube straight down the centre of the case.
These will take a few days to cure so once I am sure they are solid I will clean out the necks and hopefully work out a consistent load.
The hotmelt glue was a dead end just too soft and flexible.
Lead filled cases are very time consuming to make, but probably would be the most robust . However only shooting will tell.