Hi all Shane here from backyard choppers. Calling out of a little dot called Pahrump Nevada. Lots of flat road scenic riding and some silty clay trail riding. Also throwing $5k trek bikes down mount Charleston and turning them into 10 buck huffy’s. Hehe loads of fun. Anyway I’m a bike builder...
https://www.porkchopbmx.com/non-returnable-evolution-v-brake-bicycle-46159865.html
I think this is what you are looking for. Bolts onto the seat stays. They are pricey. You will end up having a $100 brake set up. I did see a YouTube video of a klunker build that had some made locally. Not sure...