You need cake cards. There are lots of places online you can buy them, or if you have a cake decorating shop anywhere local to you. I'd say you need a bigger difference in the size of your cakes than just 2cm. That would leave just a 1cm difference round the edge, iyswim.
You need cards under each cake and dowelling rods to support the weight. If you use cards that are each a bit smalle than the cakes you make you could carve it into the right shape then the fondant should follow the shape of the cake pretty easily. You could leave a tiny bit extra round the edges and then raise the cake up and tuck the extra fondant under the card.
I've never made a stacked cake before. Would it need some kind of support to stop it collapsing? Also how do I make the cakes look ring shaped instead of disc shaped? I can carve the cakes to roughly the right shape but how do I get the fondant to go round the bottom of the cakes.