Hi Buckie. Details: Box dimensions -- Square ones are 4in x4 od, 12 inches long, ¼ inch steel. Rectangular side boxes are 8 in wide x 3 deep by 10 in long, 3/8 in steel (overkill….but what’s a little more ballast out back???) . I scrounged a piece of 8x6 box steel and cut in half (welded on a back) to make the rectangular ones----it just happened to be 10 inches long. Added a 1/8 inch plate bottom to all boxes leaving ½ inch on both sides for water/dirt drainage. Square ones easily hold a ¼ 16foot chain, rectangular ones just hold the 5/16 16 footer. There’s space for 2 ½ inches longer side boxes if you use heavier/longer chain. (or make the box 8x4, for example): 4x4 is about as big as you can mount on the tractor side of the hitch (i.e tractor fender clearance at full lift. Check this on your 2210; be careful not to mount these too high for the same reason. ¼ inch thick material (in whatever dimension) is ideal.
Take a look at the picture posted with this, it shows the hitch sitting on my welding table during fabrication. Attaching holes in the back of boxes were drilled at 5/8 so the welds could be a bit sloppy. Welded 2 pieces of 1/2 inch threaded rod per box to the quick hitch. I used threaded rod & bolts in case the square boxes interfered with the PTO shaft. While they make things a little cramped, I’ve had no problem hooking up the bush hog with the boxes in place so I’ve never used the “bolt on, bolt off feature”. So if you’re sure of your chain size and length, you could forgo the threaded rod and just weld them directly to the hitch---it would be quicker and thus cheaper if you’re paying someone to do it.
One other fab note---I had to off set the mounting of my side boxes so not to interfer with the function of the quick hitch. i.e, boxes can stick out in the direction of the tractor tire but not in the direction of the attachment pin. Hard to describe---take a close look at my first pic and you'll see what I mean.
Good luck.