Weld the bottom 2x2 to the 21" 4x4.

Weld the two 4x4 angle pieces together as pictured, ensuring they are perfectly flush and square.

Skip around while welding so it doesn't warp one way or another.

Grind off the top weld linking the 4x4's so the bearing will be able to sit flush on it

Also grind off near the bolt holes on the bottom.