MMike, I Really LIKE your design! I don't know how much thought or knowledge was put into it, but I like to comment on a few features for those following!
Utilizing square tubing vs round gives more surface area to weld and results in a stronger assembly. As an example: If the horizontal support was round tubing, it would have to be ground to match the radius on the vertical tube and have a circumference of 4.7" to weld the joint. If 1 1/2" square tube, a simple saw cut and 6" to apply weld. Welding the square tubing to the flat mounting plate is also easier/better than with round tubing. I know we're only talking about holding a few hundred pounds, but if the rake starts bouncing, the load on the weld is greatly increased.
When I build mine, and I will, I'll have 2 minor modifications: fasteners holding casters will have nuts on bottom...only because mine is store outside in the elements. I'd add 1 possibly 2 additional holes in the outer vertical tube for a more finite adjustment. Bob