Here are some pics of my hay unroller. It is not homemade, but is shop made by my local FFA chapter. They make 10-15 a year to sell and have been doing so for 20+ years so the design has been pretty well tested. Mine is about 10 years old and haven"t had any trouble with it - feed 4x5 and 5x5 bales, but it can handle larger. On bigger bales I try not to carry them to the field with all the weight on just the spinners, but instead cradle the bales with the arms until i'm ready to unroll - figure it puts a little less stress on the weak points.
Majority of the entire frame is made with 2x3" tubing with the arms being reinforced with a piece of 1/2"x3" flat stock welded down the side. Hope this gives you some ideas.