[Christ]

You make a plug with it. You layer the Saran wrap so you don't get the foam on the bumper, then once the foam is set, you can remove the whole plug from the bumper. It's easier to do it that way, since it allows you to remove the plug for testing or hot days. Also, it will allow you to remove it for whenever you decide to layer fiberglass over it and make a real plug, instead of just using a hunk of foam.

Alternatively, you could glue a bunch of insulation foam board together and just trim it to fit in there snugly, and that will achieve the same end.