Here is the weight I just built. I was looking to get the center of gravity as far out back as I could so that when I use the FEL I'm getting weight off the front wheels rather than only adding weight to the back wheels which is the case with filling the rear tires.
I can just back the tractor up to it and the drawbar slides right onto the two pins in the front. I did it this way because I always have a rough time hooking the 3PT up to things (probably because I am a new tractor user)

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I designed it using Google Sketchup and I was shooting for 500 lbs (Quikrete is about 133 lbs per cu.ft.). One deviation from the plan is that for the top link piece I used some misc lift link I found at AgriSupply. I embedded it in the concrete and it works fine.
My biggest regret is that I lined the MDF form with plastic so the MDF wouldn't breakdown from the moisture in the wet concrete. I didn't realize that it was going to look so much like a wrinkled prune

. Maybe I'll name it Yoda.