Here's the information I used when I made mine...

These tools are a necessity for testing compression on the 7.3L Powerstroke diesel. The reasoning for that is there is insufficient clearance between the rocker arm for the angle that the glow plug port goes in for a standard straight adapter to work. It will hit the rocker arm. No fun.

The nice thing about this particular setup is that it can be used on ALL 8 cylinders. You have to mess with the hose a bit on the back cylinders, and you have to noodle past the A/C Compressor for cylinder #1, but it will fit. At least it did on my 99.5 Super Duty.

As a bonus! I've seen a few people on here have TDI's- The 7.3L Powerstroke and the early TDI's (1Z, AHU, and ALH engines) have the same glow plug thread pitch- This tool can ease getting a reading as you don't have to snake the compression tool down amongst the injection lines. Bonus!

Your starting goodies-
An economy diesel compression tester. I know for a fact that the Harbor Freight tool #93644 ( 12 Piece Diesel Engine Compression Tester ) has the 1/8" NPT adapter we will need. So I recommend it.

You'll need a 1/8" NPT female-to-female connector (I call them barrels)
You can get the just about anywhere that has a brass fitting selection. I got mine from Parker Store as it was easier to get it at the same time as our next piece.

You'll need an adapter to fit the actual glow plug port. Parker Store part #M10-1/8F8OHGS accomplishes this. It is our glow plug port thread pitch, M10-1.0, in an O-Ring boss male fitting on one side, but on the other adapts to 1/8"NPT female threads.

You can see if there is a Parker Store near you by searching at McCoy Sales Corporation - Manufacturers Representative and Distributor

If not, here is an internet alternative- ( M10-1/8F8OHGS MROstop.com )

You will need a grease gun hose whip. Not the kind with a fitted grease fitting on one end, but the kind with 1/8"NPT male fittings on both ends. I got mine from Ace, but noone seems to have the 12" whips online.
You can get them pretty much any auto supply store. Autozone should sell a 12" whip for about $4.99. I think I paid $6 at Ace. Either way, it doesn't matter.

You will also either need pipe dope or teflon tape. I won't link that as it should be obvious to anyone who got this far in the writeup.

At this point, lets see....
I paid roughly-
$30 for the compression tester.
$6 for the grease whip.
$1.50 for the barrel adapter
$12 for the M10-1.0 to 1/8" NPT adapter
And I have a healthy supply of teflon tape on hand.

So, thats roughly $50 into this venture, about $20 into the adapter. Beats the price of just the adapter from Snap-On.