you can use cake on a board, RKT, foamcore, gumpaste... really it comes down to the look you're going for and what you want to do.
I did the lids on this box cake out of gumpaste. I made them about a week in advance so they could dry and formed them around a cake board (that was elevated so the sides could dry straight) I personally wanted the look of a real lid that would slide over the cake (and yes they really did slide over the cake.)
http://cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1397586
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