One piece of 3/16th inch foamcore cut to 11x15 (or larger) will make a great board for a 9x13 cake. You can cut the foamcore with a craft knife or exacto knife and then cover it as you please. If you want to put ribbon around the edge, etc., just stack up on the foamcore until you get it the thickness you need or buy the 1/2 inch foamcore (more expensive and harder to cut, but nice when using wider ribbon for the edge treatment).
Rae
I use all the cardboard I can find. Usually I take the ones that are used for cola packaging. My dad owns a store and each time he puts the colas in the fridge for the customers he saves me the cardboards. And sometimes when he receive marchandise in really hard boxes he saves them for me also. Never pay for cardboard!! that's a good thing!!! 
Lorris
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