What can I use to split a bake pan?

I need to split a recipe in half, and that should fill a 4.5 inch by 9 inch pan, but I only have a 9 inch by 9 inch panI want to put something in the 9x9 pan to split it in half and put the mix in one sideWhat should I use? Cardoard? Aluminum Foil?

there is a pool slide

foil not cardboard it could catch on fire