Well...if you`re going to push me lol
I tried all kinds of aluminum tubes at three stores. Just felt using a round tube would weaken eventually at the point where it is bolted to the cleat.
For similar reasons I rejected steel as well.I could write a book on the subject.
In the end, I used 1/2" square steel stock. I removed the cleats and inserted 5/16" Rod Coupling Nuts into the collar of the cleat and screwed them back in to the hull. I secured the handles to the cleats using long threaded 5/16" bolts/lock washers. I secured the bolts below deck with wing nuts/large washers. Painted everything with Rustoleum and sealed off the ends with Liquid Nails. If it corrodes the project is easy to redo in no time.
The grip isnt perfect but its comfortable and very secure. Went out in rough weather several times and it felt safe having something to grip on to. It also helps you counter-balance against the boats tilting/jolting movement when getting hit with larger parallel waves...