We use 4x4 legs for our box blinds. Build a box frame out of 2x8's, install plywood floor, and then run braces under the plywood attached to the 2x8's. Two braces are placed evenly off a 3rd center brace. We then insert 4x4's into the inside corners of the box and attach with lag bolts, washers, and lock nuts.
We used to complete stand enclosure with luan walls, plywood roof topped with corrugated metal panels, cut windows and door, use hinges to hang cut out door, and paint. We have about 5 of these installed in our hunting lease with bases that range from 4 to 6' tall off the ground. We've only had one that tilted from sinking. We anchor using steel cable and turn buckle.
We built the last 3 blinds with Baronnett Big Mike hunting blind's on top instead of the luan enclosures. The tent blind's are 80" tall and their base is 59" square. Makes for a much quicker and easier build. We take the tent blind down at end of season and store until next year. They've proved durable and should last several + years. We'll just replace the tents if they get damaged or wear out.