I think I'd make up some wooden handles that slid on. Two-piece would work easiest, inletting each tapered handle into each 1/2 handle. Then, lay in the re-silicone high temp goop, and also use it to glue the two 1/2's together onto the mopulds spikes. If you use high-temp silicone to hold them. they'll last like that for years, but will be removeable without damage if desired or if needed replacement.
; With handles, you can heat the mould properly to get nice smooth casts that will be as perfect as the mould will produce and I suspect it will produce perfect bullets.