If you make a compression tester, you need to incorporate a tyre valve assembly at the engine end, to act as a non return valve, to retain the reading.

The tip of the valve stem needs to be accessible, so that the pressure can be released before taking the next reading.

Ideally the connection needs to be retained in place. You will have difficulty holding the tester in place with compression pressures above 120 -130 psi, on a petrol engine.  So a sparking plug body will probably need to be modified.

Either way, buy or make, not likely to be cheap.

And measure the pressure after the same cranking time, say 10 seconds, so that results are comparable.

For a diesel, you will need an injector body that can be modified, and the compression pressures will be high, of the order of 500 psi for an engine in good condition.

With regard to an injector tester, presumably for a Diesel, the gauge will need to read to quite a high figure.

20 years ago breaking pressures were 210+bar, and with emissions regulations tightening almost yearly, may very well be higher.

Howard

https://www.toolbee.co.uk/sykes-pickavant-33100000-universal-digital-pressure-tester-comprehensive-kit.html

You can hold a 12v injector open with a 9v battery without causing damage. I’ve wired them open and stuck them in an ultrasonic cleaner to clean them before.