I have seen blank rubber plugs used from the factory to fill the hole in the cab.
Problem is I can't remember what truck I saw it on.
Perhaps it was a 70's IH medium duty truck?
It was something that used a PU cab on a larger truck and the tank was on the frame on the big truck, and in the cab if it was a small truck.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-