In case anyone else has this issue now or in future...
I recently bought a used Mason Definition frame from eBay. It was one of the original, post-mount frames, and I wanted to fit Campagnolo H11/Chorus. The calipers are flat-mount only.
I considered using one of Shimano's original post-mount road calipers (the frame actually came with one), but didn't want to mix Campag and Shimano bits. I also looked into using a Magura or Hope caliper. Then, I came across this website: https://www.assolutions.ca
They sell various adaptors to convert post-mount frames to flat mount. I placed my order and, after two weeks, a nicely-machined part turned up in the post, complete with mounting bolts.
If you're new please join in and if you have questions pop them below and the forum regulars will answer as best we can.