I did this myself and it works like a charm...you dont need to take to machine shop...heres what to do:

1. I don't remember exactly what i did, but you disassemble the injector and throw away the guts...I think I remember enlarging the hole in the threaded portion (opposite the spray end) with a standard drill bit. Just make sure you have an unrestricted avenue for air flow.

2. Grind off the two fuel bypass nozzles....braze or weld those two openings shut. (I did it with standard mapp gas & brazing rod from home depot). Another easier idea would be to clean with degreaser, fill bypass nozzles with epoxy, and flatten with hammer to crimp shut...should easily hold 350 psi.

Done!

When using this as an adapter, install one of those heat shield/washer things for it to seal against when screwed in.
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Now you need a connector to connect the threaded portion of the injector/adapter to the compression tester. This is tough as this is not a standard metric thread (I dont think). What I did was go to a junk yard and cut off the tip of a metal fuel injection line with the threaded connector..... to connect to the injector. Luckily, standard 1/4" brass compression fitting (suagelock) worked perfectly as a connection onto the cut end of the injection line. From this I used a bunch of swagelock fittings and a standard 500 psi pressure guage to make a compression tester.

If you want to make a compression tester, you need to install a one-way checkvalve and a bleed valve in the setup, because when you check compression, you need to crank the engine 5 to 7 times in order to build up pressure in the tester. If you dont have a check valve that "saves" the pressure from each piston stroke, the pressure guage will simply go up and down with each piston stroke, and never reach the maximum pressure which gives you the compression reading. Once you take a reading, you need to bleed off pressure by opening the relief valve, then close the valve in prep. for next test.

I bought the pressure guage from Grainger, and all of the
other fittings (brass) including check valve and bleed valve, from local Swagelock distributor, over the counter.

Note about that check valve....they come in various specs. and sizes. Obviously, you need to size threads to fit the rest of your setup. The important thing is the pressure ratings....they have a "breaking" pressure, the amount of pressure it takes to break the valve open and allow air through. This you want to be low (1 psi) in order to allow unrestricted flow from cylinder into the tester. Then there is the "holding" pressure..how much the valve will hold...you want this to be above compression pressures (500 or more psi). I dont remember the exact pressure ratings of the one I got, but I belive the breaking/holding pressures were 1psi/1000 psi, or something similar.

I will post pic if anyone wants.

The whole setup cost about $60, fun to make, works like a charm, better/cheaper than those on the market.

Good Luck