Lioness has it. Pot the fastener holes with epoxy as usual. Drill to tap drill size, tap the hole, mount the fitting and fastener using the epoxy threads, go below and mount the backer and nut.
The Gougeons claim the epoxy threads are plenty but I like the security of the nut as well.
Note: this pretty well has to be done as an original type install - it's not something that will work to well on an underway repair. In a case like that Ish's method would be better.
raced on a boat with winch bolts threaded into epoxy. had two winches break free and lift an inch off the mounts within 5 minutes of each other.
couldn't get a backer and a nut on. there was absolutely zero access. the epoxy was drilled out, repoured, redrilled, rethreaded, and the winches remounted.