How about some 4' X 8' panels using 2X4s with wire stapled on them and then simply tie the corners together (I tacked a triangle of plywood on each corner for a gusset). Then throw some deer or fruit tree netting over the top to keep them in and a tarp over one end for sleeping / shade area. This would allow you to go as big as you wish, move them around if you choose, it's cheap, and easily disassembles and stores for winter. Shove a heavy tree branch thru the wire about 2' up for a perch and you would have a couple happy birds.