Boring out the hole in the hub of the flanger to fit around the center post of the base. The cylindrical thing on the far right is an oil-impregnated bronze bushing that will bear the radial load of the upper assembly.
I built this liftgate extension flap (in blue) for a freight truck. It folds down flat to form a larger platform.
My roommate giving my other roommate a ride to work in the cargo-coffin.