I only want to do this once for the house.  I have tired the plastic backboard on the plastic sand filled unit.  Blows over, breaks, last 5 or so years.

Going with a 4 x 4 sq. tube, I think, and a 4 ft long x 3 ft tall back stop.  Going all steel. 

How thick should the back stop be?  Do not want to do any bracing for the birds to next in.  Just want to weld the 4x4 on the back.  Prob. make a 90 degree bend/angle in the 4 x 4 so it sticks out over the drive by 2 or 3 feet, so the 4 x 4  would be welded in the middle of the back. 

3/8 inch to thick?  lol