heavy duty tripple knuckle joint

I am trying to build a horizontal swing arm, for explaining purposes The current knuckle joint design has a single end point that is sandwiched between to other circular end point. What I am looking for is to create a double jointed interface instead. Each end will be welded to a square tube the moving arm requires heavy load tolerance when extended. see picture for question please. I need help designing this and also figuring out what thickness the square tube requires.