I often trace a template of the bottom of the topper, cut out the cardboard just slightly larger, place 1-3 dowels (depending on size of topper) through the top tier, set the cardboard on top of that, the topper on top of the cardboard. I don't necessarily secure it to the cardboard, but little dabs of royal icing work well.
Tina
I wouldn't worry about using a cardboard circle under the topper unless it's really heavy or the bride doesn't want icing coming in contact with the topper (like if it has lace around the base of it). I usually just lightly press the topper into the icing once I've delivered the cake and have it set up; that's always been enough to keep it in place for me.
(PS: If the topper is very heavy, like pewter, or breakable like with glass, I would try to convince the bride to consider having the topper placed on the table next to the cake instead of on top of it. The last thing you want is for a heavy topper to go toppling down the side of the cake.)
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