I just used a screw with an oversized head in the channel. I wasn't sure how much room there was when sliding the arm up, so elected not to go with a plate. Having a domed nut on the outside face though means there is no nut in the channel (and finishes it off neatly). There is always the risk of it pulling through I guess, but I felt the awning post thing was strong enough, and it was effective sandwiched between the flat base of the eyelet thing on the outside and screw head on the inside, and there are two connection points/screws per eyelet.