Hmm I see from my ebay history I bought two different clamps initially for the 7x10 carbon seat. Specialized and trek/bontrager.
But later I ordered for round rails, and that was this item here: https://www.ebay.com/itm/202604990703
For Trek/Bontrager. And that is the clamp I'm using now with the fizik seat.

Hello

Glad to hear you like your bike.
I've built one this year as well. And I had no previous experience. Watched a lot of velobuild's videos on youtube that inspired me to build one.
Some things came easy and some took a bit of research. When tightening the headset nothing would turn. Had to buy shims to put under the bottom cap. Didn't know there were supposed to be something there, and none came with the frame/headset.
Also massive problems with seat post. Ended up with some electrical tape around the seat post and making it as tight a fit as possible.

@jcr: The cable housings stop at the entry point. I was thinking of drilling a bigger hole in the inserts and just run the housing thru, but opted for open wires inside the frame.