Open air could be as simple as a pipe stall mare motel. Our Barn that we are currently designing and building is 44x48 feet (covered area). We have 6 16x16 stalls with a 12 foot breeze way. Two of the stalls we will enclose and make a tack and hay/feed storage. The solid portion of the stall walls are about 4-5 ft high then it will be open to the roof line. For the west side stalls mostly we will put up shade screens to block hot afternoon sun (110-120 degrees) but allow air flow.

As far as the sand each stall will have about 8 inches of sand. The great thing is when you pick up the poop the sand falls back to the ground and the urine such soaks down through. Shavings there is a lot of waste and they tend to spread to the outside edges of the stall and your horse is back down on hard ground. You are also constantly replacing and striping stalls. Straw just attracts tooo many flies and is a pain in the hinny to clean. Your initial cost of the sand may be a little more but you should only have to add sand maybe once a year a most, if that even. We have one horse that drinks and urinates a lot and we have to clean his pea spot out real good about once a year and add more sand. The sand we get is called "reject sand" it is not super fine but a little coarse. Kind of like the stuff they use in playgrounds. It was pretty cheap here, we buy it directly from the gravel yard.

Also, you can wet it down in the hot summer and being in the shade it provides a nice cool place for the horses to stand and lay, especially if you have a fan on, mine love that. It is great also for older horses who lay down alot, horses with navicular or laminitis issues. The sand moves to where the horse needs the most support.

Some people worry about sand colic, but here in AZ being desert we feed Psyillium every day since there is not a lot of grass pastures and most horses are out on dry lots. So if you are a little worried just feed Psyillium. I have never had a problem with ours.

Hope this helps.