A visit to a place that makes trailers will probably give you what you need. If not buy a piece of 1 3/4" rectangular tubing and make your own. Your 9" has either 3" axle tubes (car) or 3 1/4" (truck
). A 1 3/4 x 4 will let you trim out stock height saddles or you can raise/lower the rear with taller saddles---up to 1.75 x 8. If saddles are on top taller ones raise and visa versa for the bottom.

Evan