The diameters of the rotor discs that have been tried are 140mm, 160mm, and 180mm. The caliper still doesn't fit.
Depending the frame. All disc sizes need an adapter but one the frame is built for.
I had a similar issue with my first build, as I didn't know an adapter was needed.
But here it is very strange.
Do you have a two piston caliper you could try to fit there ?
Should the actual brake mount be on the seatstay and not on the chainstay?
Most frames here, if not all, have the brake mount on the chainstay.
I think having read recently that for carbon frames the generally tiny seatstays aren't strong enough to handle the brake power.