Just a regular round spacer/washer/ring, whatever you want to call it (see picture).
As I said, i think that 0.5 would be enough, and searching for thinner spacers I found this from FSA https://www.tradeinn.com/bikeinn/en/fsa-spacers-kit/138244357/p
look at the description:
 "Integrated headsets sometimes require micro-spacers to adjust the depth of the machined headtube in order to ensure correct fit. Micro-spacers should be used between the top cover and compression ring to make sure the top cover does not contact the top of the headtube."
That's exactly what I did, so, from this description, I assume that this is not a problem of this particular Hygge's frames, it's a common issue. Also I just realized that is enough to install the ring as it comes, not necessary to cut it into a C shape. I'll try some day which thickness is the perfect one, for now I'll stay with the 1mm as it works.

Did a 125km ride today and the play did not come back. So for now the 9Nm on the expander (higher than recommended) and the 5Nm on the stem bolt seems to hold.

How far does your seatpost go in? There is a max insertion depth, so it is normal you have to cut your seatpost

Not many people post photos of their completed bikes on here, but I always enjoy the posts. So I wanted to post my recently completed build on here, and share how much I spent. A lot of builds on here are really balling on a budget but I really purchased from mainstream vendors for less sketchiness, build came out to $3,288 assembled by my LBS

Mechanics first "Chinese" build at the bike shop, he was very impressed with the quality and noted that the internal cable routing gave him no problems. All of the workers at the bike shop where really interested in the build and none had ever heard of Winspace. This really shows how obscure this forum is, we are so far away from mainstream. He did ride it and was super impressed with it, I told them how my goal was to create an entry level roadbike, but have carbon wheels at the same price point.

8.1 kgs with everything, 17.85 lbs

Frame: Winspace T1550 ($1230 Black Friday)
Wheels: Winspace 2023 Hypers D45 ($1049 Black Friday)
Bar: Winspace Zero SL 380 W 105 L ($171 Black Friday)
Groupset: Shimano 105 R7120 Disc ($688 Merlincycles)
Tires: Conti GP 5000 ($130)
Tubes: Ride Now TPU 36g ($25)
Seat: Specialized Power Comp (Free: Old Bike)
Pedals: Shimano 105 SPD-SL (Free: Old Bikes)
Shop Assembly- $240