One option to consider when building a tester is that you don't have to install the hose barb on the intake plug.
Take an old spark plug and weld a 90 deg pipe thread elbow where the spark plug turns into steel (break all of the porcelain insulation off so it's a clear shot into the engine), then screw on a steel hose barb (or quick connect if you have a good air seal in there).
Install the stubby spark plug adapter, hook up hose, and plug the other openings.

A bonus to this method is that you can also use the same plug adapter for a compression tester.
I use a similar method for differential compression tests on my radial, except that I welded 9 plugs up by installing a quick connect on the ends of every one of them.
It's important to check compression while the engine is hot, and as you could imagine removing all 9 plugs, then installing and removing an adapter for each cylinder can cause the engine to cool off before you're done.
Had the same idea for my hybrid, but I realized that there would be clearance issues with the straight plug and hose so I welded mine up with a 90.
Works great with the limited clearance!