4 x 4 with a little work to nestle the barrel into it. Then some hose clamps.
Heck, if you have the stock from a percussion or flintlock single shot, hose clamp it to that and just hand hold it. Only .45 cal.
ADDED:
In fact, if you have a semi-auto, field strip it and you could likely hose clamp the barrel to that. Get you all sorts of looks at the range.