I have a Walco brand model 495 (48"X48"X18") dump box that I got with a 5' brush hog and the L3240 tractor.
Before I had any other implements it was my primary ballast choice.
I'd just toss as many rocks as I needed in it before using the FEL for any heavy jobs.
Worked great for that - only drawback is it's kind of bulky - but compared to the mower, it's downright compact.
Also used it to move a lot of stove length fire wood. Between it and the FEL I could take a decent load in one trip.