Personally, I'd use cross bracing of some sort if it was only 30" off the ground, just for stability. Higher and I would use bracing no question.
I used 3"x3" square tubing I had on hand for my 8'x8' blind, it is 14' high. Poles are cemented in the ground and I also cross braced using some 3/4" round rod I had. I like stable and it is solid as a rock. My blind is wood, but the rest of the frame is all steel.
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