I am going to design it around the 70# Deere weights. I want to be able to add at least 300# to the box blade.I think that's a good idea. I have a couple ballast boxes, and one has a small hitch on the rear of it, though it is the draw bar type instead of the receiver hitch like yours, which is much more versatile.
For the weight bracket, I have thought for some time that a teepee welded on at each end, with a cross bar made of 1/2" X 4" steel going from one side to the other would be sweet for loading up suitcase weights easily. A tab with a hole in it welded to the outside of each teepee could accommodate a rod to run across the length of the weight bar through the trough made in the suitcase weights at the front (at least like the ones Deere sells in 42 and 70 lb sizes) to hold them on, and a center support or two could run down to the braces that run front the front of the box to the rear at the top side for added strength.
I almost did the same thing with my BB. Then I went with an Omni Hitch with weight bracket so I can move gooseneck trailers also.Put a receiver hitch on the box blade.
Next step is to build a weight bracket as the BB is going to double as rear ballast for the FEL.