Unfortunately, it looks like the Cobra, with the integrated stem is a bit different to the standard Mk3.
Looks like it's designed for a fixed length of headtube + headset stack, with no adjustment.
The steerer slims down to save some weight, maybe 30g, but causes you some hassle.
As you know, the standard Mk3 has an adjustable top section on the steerer, which screws up and down to preload the headset and spacers.
The Mk3 fork was supplied with a selection of spacers, including one long enough to bridge the gap across that chamfer.
To sort yours, you need a custom shim made that will take up the slop between the headset and steerer.
It would need to be a 2-part split shell that is then bonded onto the steerer before it's inserted through the headset.
It's nothing structural, just needs to make op the space and not fall off into the headtube.
Fairly simple turning job: 28.6 external, 27.1 internal and around 30mm long, split axially into 2 parts, I could make that for you:
You could then use standard 1-1/8" spacers to fill the stack height up to the base of the stem if the stem has enough float to compress down the steerer.
If not, you'll need an adjustable spacer to pre-load the headset. The ABR Float (around £
, comes in a couple of length ranges and would be in keeping with the CNC look of the fork:
All the best,