If done right even 1/8" would do the job. It's just a matter of the sleeves being the right length. Personally I drill a 1/2" hole all the way through the frame and then drill the outside with the larger size for the tube to pass through. I slide the tubes in and put a few heavy tac welds on the edge of the tube holding it to the frame. Then plate both sides, effectively trapping the sleeve in place between the plates.
