[In The Ten Ring]

What are the dimensions of this hole?

Assuming it is big enough, you could fashion two steel plates to bolt into each other and tighten up, sandwiching the firewall between. Use pan-headed machine screws (actually one would do) and put the head of the machine screw on the inside (of the cab). You could use some sealer around the edge or even some foam gasket material from Lowe's to keep out moisture.