When using pans that are 12 in. or larger, we strongly recommend using a Heating Core. It will bring heat to the area of the batter that gets the least amount; the result will be a flat, even cake. Grease and flour it, inside and out, place it in the center of the pan, fill it to the level you would fill the cake pan, then fill the cake pan to the proper level. After baking, remove the core, remove the cake from the core and place it in the hole. After icing, no one will ever know.
Please note: Standard home ovens will only accommodate pans that are 16 in. or less -- even though they may be a lot wider, the internal depth is limited to this maximum. Wall ovens are smaller than standard ovens, so check the inside dimensions before making your purchase.