Wood. Open (er, with at least two sides closed for a wind break).
Is your horse pastured with other horses? If so, definitely leave at least one full side open (or two half sides) so if your horse gets chased in there its easier to get out. Leaving it open is also more welcoming to your horse as it seems "bigger" since horses are naturally a little claustrophobic. If you want a tack room, what about this? http://www.alansfactoryoutlet.com/Po...esized-600.jpg
Or, if you want somewhere to enclose him in, this (if there is more than one horse in his pasture close the stall door, if there is just him, you can leave it open) and use one of the stalls for the tack room. http://www.penndutchstructures.com/m...g-SM%20png.png
Also leaves a nice space where you could put a tie ring/cross ties without being worried about being trapped against a wall by your horse o.o