How thick is the metal? Harbor Freight had a couple of cheap punches for what they called light metal (I am guessing tin/aluminum) and starting with leather.
I bought a set, but they are cheap, so steel would be right out from what I can see.
The other option is to machine a punch to spec, and if it is something you think we could sell to other brewers, we could have our first Hack Factory product to sell.
(ok, back to reality) the whole punches I have seen are just tapered to spec, so even with our nicely refurbished metal lathe, we should be able to cut a stainless steel rod down and then hollow out a core.