You could decorate and arrange the cupcakes on a card board separator cut to fit slightly smaller than the overall flower outline. If the cupcakes are in liners, attach the cupcakes to the board with a little double sided tape, so that when the cupcakes are lifted off to be eaten, the bottoms are not covered with frosting. If the double back tape is too sticky and will make getting the cupcakes off difficult, it can be "degummed" partially by pressing a lint free cloth against the tape briefly after it is on the board. (I do this all the time with bandage tape so it won't pull out all the hairs when removed!). Alternatively, they could be attached to the board with RI, especially if they are not going to be in liners. Then place the cardboard on top of the frosted sheet cake. place dowels to support if the cupcake board is heavy. Pipe decoratively around the edge of the cardboard if needed. Just a thought, HTH, good luck