Take a scrap chunk of wood about 4" x 4" x 10" or so, a piece of oak is best say a scrap from a pallet, and bore a 3/4" hole in the middle of one of the 4" sides all the way through. Drill it in the middle of the 10" length. Then using some kind of saw (I use a table say, but a jig saw, hand saw or band saw would work fine as long as you remove a 1/4" thick slice of material) cut the board along its long lenth so that the cut falls smack on the middle of the diameter of the hole. It need not be a nice cut at all. Cut the whole 10" lenght so that you end up with two 10" x 1 7/8" wide (you removed 1/4" of material, remember) with a little less than half of 3/4" hole in the middle. Put each half around your barrel and you can then put the wood and the barrel in a vice, or in a pinch use big C-clamps.
You've just made a (nearly) FREE barrell vice that won't mar your barrel!
efxguy