Hello Dave, here are the pics of the receiver we built on my 720.
The goal was to get it rearward as far as possible to obtain maximum lift capacity, so we put it on the lift arm cross-bar. We sandwiched the cross bar with 3/8" stock to transfer the twisting forces as close as possible to the main lift arms. A piece of 3"x 5" angle for the top & one side, and then a piece of 3/8" flat stock for the other side. We built it tight--no wiggle room at all-- and then pinched it together at the bottom with five 5/8" bolts. The front of the receiver is banded for strength and is positioned to just barely clear the back side of the bucket. The back end is gusseted with two pieces of 3/8" stock. Lots of room in the back, so length there doesn't matter.
Break-out force of the 720 is listed as 2000 lbs. I believe I can lift twice that. Even more on the tilt. When I got my Cat V60D forklift buried in soft ground this spring, I was able to lift the back or it high enough to get some wood under the wheels. DieselSales.com spec sheet says the V60D weighs 10,100 lbs. I had a piece of 2" solid hitch stock, about 18" or so, removed the drawbar pin from the forklift and stabbed into that cavity with the square stock in my receiver. The boom lift would not budge it, but the tilt did pick it high enough that I was able to get a piece of 4x4 under each wheel. Estimated 4000 lbs. lift. This was with a 400lb auxiliary counterweight on the back of the Bobcat.