Here is the DYI version of the DIY valve extension that is in the Lycoming SB 388C. Never fear the fact that it is made out of a 1/2" piece of CPVC pipe. It only has to grip the valve stem and withstand the pressure of the dial indicator spring nothing more....
Here is the DYI dial indicator holding fixture per the 388C. This one is 3D printed from glass filled nylon and works great. All it has to do is hold the dial indicator steady. It requires a washer to keep it pushed over against the rocker arm shaft boss. This is per the manual. The screws can be hand tightened to hold it all in alignment. The key is to ensure that the dial indicator is parallel to the rockers and at a right angle to the valve extension and the dial indicator is centered on the shaft of the extension. It is easier than it sounds to get it all lined up.
From the side:
Per the SB 388C, the dial indicator measuring point is 2.5" from the top of the valve guide.
The test is simple... push the valve in a little to get it off the seat then take your finger and press down on the valve stem right where it goes into the guide. Set the dial indicator to 0 then without rotating the valve (rotating screws up your 0 point unless you take the time to ensure that the extension is perfectly concentric with the valve stem) take your finger and press up in the valve stem right where it goes into the guide. Take the reading...repeat a few times for accuracy.
Note: Unless you have the latest version of the Lycoming rocker arms, they are not interchangeable. Lycoming SI 1454A details how to ID them and where they go....