I get my at Rural King or Tractor Supply.
They are called 2 1/16" X 3/8" Square Axle Mounting Bolts,
to fit over a 2" X 2" square tubing..........
I weld on a plate of steel with holes to the underside of my crossbar tubing where it intersects the trailer frame. That gives me a good tight connection without needing such a long reaching "U" bolt.
I also open up that 2 1/16" to something a little wider to allow room for the nuts to fit better.
I also sit the crossbar on top of the frame instead of hanging that crossbar from under the frame.
When repairing a trailer, I use thicker tubing. My Holsclaw trailers where 16 gauge tubing (1/16")
and I upgraded to 1/8" or 3/16" depending on if I plan to stack another boat on top.