Actually, I plan on making nice blockoff plate for a few of the guys down in Texas soon, unfortunately ive been dragging my feet on it, but I promise to get them done.
get a piece of sheet metal the thickness you desire then weld a stud (could use a bolt) to the back side of it at the corners where it comes in through the cab. on the back side, put a piece of strapping across it and tighten. make sure to put some sort of gasket on it. no need to drill holes in the firewall, etc.
Actually, I plan on making nice blockoff plate for a few of the guys down in Texas soon, unfortunately ive been dragging my feet on it, but I promise to get them done.