I think that people make a hollow mount and then just have a little bit at the end. Like the paragon boss, e.g. BK0030, except you don't really need 15mm. Ceeway sells some paragon parts, but maybe not the flat mounts.
https://www.paragonmachineworks.com/...er-mounts.html
When I went to look for their mounts, I noticed they finally sell front flat mounts. But they are for the rear. I understand that's fairly common on custom bikes nowadays, you might want to look at doing that.
https://www.paragonmachineworks.com/...t-dropout.html
The fixture to place the front mounts on the rear isn't that expensive.