That's good that you've got it sorted.When is a 37mm seat clamp not 37mm? (when it is Orbea).
A 36.5 seat clamp from a Trek fit perfect.
My mechanic suggested this as Trek also uses a 31.6 seatpost on their carbon mtb frames.
It's strange that Orbea would be so unspecific about the seatpost clamp sizing saying 37mm diameter (also Hypersprite's post above where the part was labelled as 34.9mm on the order for a seatpost clamp that isn't 34.9mm diameter and wouldn't fit if it was 34.9mm diameter) when it isn't. I don't know why Orbea would do that.
The Bontrager Trek seatpost clamps are 36.4mm according to the website:
On that Hope aftermarket seatpost clamp link the available sizes are Size 28.6, 30, 31.8, 34.9, 36.4 and 38.5mm diameter so those are likely to be the most common seatpost clamp sizes for the vast majority of frames I'd have thought.