Personally, I'd get those steal supports for the platform that they sell on Sportsmans guide for $80 then build a box around that. They hold 4x4s and are on a slight angle for more stability. Personally, if its got walls and a ceiling and you are using it for bow hunting I wouldn't go smaller than 4 feet by 6 feet. It will get snug in there at 4 feet by 4 feet. Also, be careful getting that high up. I hunt pretty high with my hang on stands (18 to 25 feet) but when it comes to enclosed box blinds, 12 feet is a lot higher than you think. Plus, trying to get a box blind up that high is not fun unless you have some good farm equipment. Good luck and let us know what you decide to build!