I used the broken-off bracket from a water bottle cage as my first booster plate, for a Weinmann caliper on my UO8.

As thin as the flimsy bracket was, I still had to use longer bolts iir, and it worked perfectly after I adjusted the hole centers with a round file. This after carving some extra tire clearance into it.

Since the booster plate doesn't have to sustain any bending load at all, it can be effective even if very thin and carved away.

I made this one out of 3/32" aluminum, and even as tall as it is the rear braking is night and day better than before I put it on. It's for cantilevers, but you get the idea:



I sawed the big eccentric adjuster hoops off of the lower ends of this aftermarket brace, making it shorter but still clearing the 700x40 hybrid tire. It's secured by a washer sandwich.
It's also quite effective. Get a load of how much metal is supporting both ends of the canti studs!: