Thanks, Geo. Your radius arm restraint strap attachment looks like a good one.
Your mention of a washer reminded me of another trick when putting a sleeve in a very thinwall square or rectangular tube. Place a washer with an I.D. slightly larger than the O.D. of the sleeve on each side of the side of the tubing and then weld around the outside and inside of the washers. The diameter of the washer spreads the load over a larger area of the thinwall tubing, giving it more bearing strength. This works for thin sheet metal, too.
Regards, Neil Tucson, AZ