I would be tempted to use epoxy and ss all-thread. Should be pretty easy to source some black cap nuts. I’m using the simpson epoxy for setting rebar and all thread in concrete. the twin tube guns are a little pricey for a decent one though. You probably want this version if you go with their producthttps://www.strongtie.com/epoxyanchoringadhesives_adhesives/set_adhesive/p/set#ProductDetails