We framed up a front wall and door in a 10'x10' stall in our 12'x40' lean to shed. Once we sealed off a wall between that stall and the next one, we sprayed 3-4" of the spray foam insulation (had a local insulation company do it) on all four walls and the underside of the roof (it sticks to wood or tin just fine) and put OSB around to 4' high to keep cattle from damaging the insulation. We placed a 18,000 BTU air conditioner and mounted a couple of fans up high and it does great -- stays around 60 deg., which is the lowest setting on the air conditioner. A Cool Bot would let us make it pretty cold, but 60 degrees and dark seems to grow enough hair for smaller jackpots and county fair.
It has a dirt floor that drains to the right side of the room and seems to work fine. Fresh air can come in through small gaps under the walls, so no need for a vent. We bed it with cedar mulch on the bottom and shavings on top and can get about 10-14 days before the bedding has to be replaced so long as it is picked a couple of times per day and stirred well each evening when calves are removed. Total cost of project with $450 air conditioner was about $1800. It fits two fat steers comfortably.
As a bonus, it can be used to hang meat during hunting season.