Phantom309:
I made a carryall and it is on the tractor almost all the time. As you suspect it is handy for a wide variety of jobs. I scrounged 3x3x1/4 angle iron and figured out the dimensions based on the width of the tractor, and how much steel I had. I put the lower pins 27" apart but they are a bit wide; I would go for 26". The top hole is 17" above the bottom two. It does tilt as it rises, but just a bit. You could maybe reduce that to 16" or so.
For the floorboards I used 1 1/4" rough cut oak, since I use it to carry a lot of rocks.
One thing - it ended up pretty heavy, around 250 lbs. It's a bit tough to wiggle around when hooking it up. I think you could use 2x3 just as easily and save about 50 pounds, and a few bucks too.
Jim
PS I meant to add - if I shorten my top link, the position isn't too different on the ground, but it does tilt quite a bit when you raise it. I think this is partly because I have pins fairly low on the implement, so it rises pretty high up. So maybe the 17" spec would be good for the round bale job, if you just shorten your top link when you are doing that.
PPS I still mean to put some kind of screen on the back, so stuff doesn't fall forward toward the tractor. Some kind of sides would be really good too. I have used it for firewood and it tends to roll off sometimes.