Hi, I have that rubber boot, and slit the side at the top to make it fit and will use a larger piece of sheet metal with an oval hole in the middle to cover the whole hole and anchor the boot. A couple mothballs under the truck discourages critters.

Well, my time went so quickly,
I went lickety-splitly
Out to my ol' 55.
As I pulled away slowly,
Feelin' so holy,
God knows, I was feelin' alive..... Tom Waits, Ol' 55