Could it have a lock-nut with an O-ring on its base to make the seal to the spark-plug hole (like a clutch cable adjuster nut)?
This way, the adapter could be positioned any way the user wanted, but the lock nut would put pressure on an O-Ring to provide the seal to the head. Then, the 90 would also have to have a flat surface for the Compression tester O-Ring to seal to. A drilled and tapped hole in the side would not do.
And now a question for everybody:
If something like this were available, would you want one?
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