compression test question

You have the right ones. They are called umbrella for the exact reason you noted; they are basically just deflectors.The first time a valve opens the seal rides up and stays there.What you need to keep in mind is that at about the time the piston is pulling the hardest, the valve is nearly wide open and the deflector is sitting right where it needs to be.
To use the positive seals with the band clamps, the guide bosses would need to be machined.But they will really dry up your stems.