I built mine with no sides at all. Makes it easy to carry long stuff. I mounted hooks on the front of the headboard and under the rear deck and keep a length of small nylon rope which works great for tying down loads of firewood, which I stack crosswise in two or three rows. Hooks visible front and rear here:
I'm going to have to give some thought to making one of these and the way I handle my work. I have a 4n1 bucket that is on my tractor maybe 20% of the time and it is quite heavy. Most of the time I keep my forks on for toting things around often up to the maximum lift capacity of the FEL. I keep my backhoe on almost all the time, mostly for ballast but I also often find a handy use for it and it's thumb from time to time so it is not just dead weight.
I really like the idea of being able to carry stuff though and lift from the rear as well - I could incorporate ballast into the box as well (BTW my tires are loaded but they get really light with the 4n1 on and the BH off).