Whatever it fits in, really. Just make sure its well wrapped and protected, and not touching the side walls. But shipping 1 bottle is not particularly cost effective, so if this is for a trade, it might be worthwhile to try to expand it a little. That, or just drop a ton of extras on the other guy and blow up his porch.
They make single bottle shippers, but unless you have one already on you, what's the point? Just use a bigger box and send extras...
