Some inexpensive gauges have a tapered rubber that is BIG enough for a Model A.
I just remembered that I have a Diesel compression tester, with several adaptors, I "wonder" if one might fit??? I "think" I only used it once, on an old VW-Rabbit Diesel. About ALL I know about Diesel is that they STINK & CLATTER. Chief LOVED them, but he wuz kinda' WEIRD & FUNNY!
Bill W.