Personnally I would put the dummy on a board and on the top. Putting it on the bottom would make your cake top heavy and harder to carry. You need to have the wieght at the bottom for it to be stable. Cover the whole cake as normal in ganache. Also if you haven’t covered a barrel before I would read up as much as you can… Are you doing the wrap method?? One more thing. I noticed that you can’t just use glue for sticking polystyrene, it melts it. It must have something to do with the acid in the glue. If you decide to put it on the top just use chocolate as glue.. Good luck, hope it goes well for you.