That C-ring will cause you trouble. It too open and too stiff. You will have a bad time, guaranteed. It'll never feel tight, and it will start to creek.
Please offer a better solution SP Cycle.
OK, so this is true. I have the Velobuild GF-003 (very similar I guess) and it is impossible to tighten the headset because there is too much room between the C-ring and the fork steerer tube... Could also be that the C-ring is too open. There is also significant amount of space between the c-ring and the headset cover:
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What's the solution to this problem? There remains a very small amount of play while using the front brake.

There are some very thin spacers left over, but I can't see how they would solve this problem.
UPDATE: I bought the DEDA Nylon 270 degree compression ring. Hope this can solve the issue. Should I make this ring thinner?