I chose to put my effort and funds into a heavy implement. At least I can use it for other purposes. I'm not sure of the value of a ballast box that only serves one purpose.
Mine serves more than 1 purpose:
Ballast
vertical PVC Tubes to hold tools.
I bolted a trailer hitch to the underside before filling it
It also has extra Cat 1 hitch posts bilaterally in case I ever need them
To fill it, I stacked up old brake discs and then inserted several different diameters of PVC pipe/tubes down through them, and poured high strength concrete around them to fill the box within 2 [or was it 3] inches of the top edge, which also left a reasonably secure storage area for my chains and hardware.
I also inserted 4 J-bolts vertically in the corners, [threaded ends protruding about 4-5 inches above the edges of the box] in case I ever want to add a platform over it, or to cover it.
BTW, although our little
B2320's FEL doesn't lift much in the overall scope of things, it was always very able to lift enough to make the rear wheels very light, even after we filled them with RimGuard, but since adding the ballast box, it has never felt even slightly light in back.
Oh, and the [empty] ballast box was "free" as part of the negotiations made when we purchased the tractor, mid-mount mower [which I now regret], and FEL [which does not have a quick attach, which is another thing I regret].