In going to be making a set of double gates. I'm thinking of a steel frame with wood in fill. Each gate will be about 4 feet wide and 7 feet tall. The post on one side will be bolted to the house so not a worry. On the other side the post will be free standing. What dimension of box section would I need to ensure it doesnt defect? I was thinking of 50mm box section for framing out the gate.