Im in the middle of building something similar to what your talking about. What I found was an old grain wagon that had the ol wooden box on it and ripped that off. Then I have bolted 2 4x4's to this frame to mount my blind on. My blind is 6x8ft and 7' tall. I built it out of 2x4's sliding windows and plywood which will end up being sided and then insulate the floor and walls. I have all the sides built at this point and the 4x4's on the trailer but have had to replace a couple of the tires and bearings so still plenty of work to do on it. I wish I had some pics of it to give you an idea. My buddy did one last year and made his 7x12! Its large and the nice thing you can just pull it to whichever spot you want to sit. Pretty handy!