My spatchcocked turkey is too big for my 18x13" pan

Look at altonbrown.com. He roasts his spatchcocked bird right on the oven dish with a pan of stuffing below.

I've used foil folded so it's 6-8 layers thick to extend the pan under the same circumstance. The foil needs to be beneath the turkey in the pan, then extend past the part(s) of the turkey that hang over the pan and folded/crimped up to divert juices back into the pan.