Note these are not product recommendations, these are examples showing the features you would need on a part to do this task. I can't find one item with all the suggested features, so you may be up for modifying existing items, or making one from scratch.
Also note that most/all of these will make wheel swaps harder in that you probably can't fit an inflated tyre into the frame.
There were items in the past called Brake Boosters, that worked with Cantilever or U brakes to support the end of the mounting posts and to stop outward flex.
Problem here is they're only bolted to the end of the boss by one bolt, whereas for braking performance you would want the two ends of the booster to go over the mounting posts, just like a V/canti brake arm.
So you need much thicker metal too - a lot of what I could find was stamped steel or aluminium, which would have too much flex for a brake. These seem to be from the BMX crowd, perhaps replacing U brakes. Contrast with this casting that is easily three-times thicker, and has a top hole intended for a caliper brake bolt:
Here's another similar one from the side, but this one clamps around the fork's tines/seat stays, and does not use any bosses. Since you have bosses it would be best to use them.