Bolting them through the hull is the best way, but a backing plate is still a good idea. I made backing plates out of 1/4in ply and shaped them back to fit the curve of the hull.

I glued the foot braces to the backing plates with epoxy and then glued the plates to the inside of the hull. Some people don't think glueing the foot braces in without bolts is wise, but I haven't had any problems on my two boats over the last three years.

The concern is that it is hard to get a good bond between the epoxy and the aluminum footbraces. What worked for me was to "wet" sand the gluing surface of the footbraces with unthickened epoxy to expose fresh aluminum to the epoxy before it could oxidize in the air. Then I made an epoxy glue and attached the footbraces to the backing plates on my workbench. When that assembly had cured I epoxied the whole kit and kaboodle to the inside of the hull.