Hey Doc,
So about your construction questions. Its looks like a pretty good plan but, if your going to lay the top 2x4 flat and double them up, you shouldn't need to make them 2x6s. Your going to need the cross bracing but instead of using the 2x4, just get a piece of plywood and cover the sides. If you don't want it on the outside, stick it on the inside but make sure you nail the side wall to the front and back walls like crazy. You would probably want some cross bracing on the back and front as well. You wouldn't need to cover the entire wall, just have 2 rectangles that span from one stud to another. Best to have one in between the front doors and one somewhere on the back where it's not in the way of your plumbing. One other concern is the sag above the doors. In homes above windows and doors you have 2 2x8s across the gap to support it, but since you have a double top plate right there and are only going 2 ft you can use 2x4s. With all those mods you would have an invincible stand. If I had a dream stand, thats how I would make it except I would stain the hell out of every inch so it wouldn't get hurt by moisture. If you have any other concerns, just let me know. I hope this helps.