I make bbl blocks to hold round bbls on CF rifles from nothing more than maple hardwood.
Chop or drill out the center betw the two halves as already explained above.
You can carefully fit the two halves to the bbl shape or more easily, I just use epoxy glass bedding mtr'l to bed the bbl into the blocks leaving about 1/4' or so space between the 2 halves.
I use car wax on the bbl surfaces so that when the epoxy is set, a simple wack with a mallet drops the two halves free of the bbl.
Place a mark on the front or back end of the blocks to show which ends are which and also # them in some manner. The # will help keep them matched up if and when you end up with several pairs and they are scattered around the shop shelf as they are here.
You can usually re-use a set made for one bbl again for another bbl of another make. It may be slightly repositioned on the bbl but the contour will sometimes match saving you making another set.
I have a bbl vise for these,,taking a Mauser 98 bbl off in a bench vise is a not gonna happen type thing generally.
But taking M/L breech plugs out and resetting them can be done in a large bench vise usually.
As Mr Roller says, they don't need to be set up with torque you generally find them done with.
I use a dusting of powdered rosin on the bbl and in the jaws. Sometimes I wrap a finished bbl with a single wrap of clean newsprint inbetw it and the blocks along with the rosin.
I usually place the assembly at a very slight angle off from horizontal in the jaws of the bench vise. It just seems to me to give a better hold accross the blocks, but maybe it's just my imagination.
Straight on horizontal hold will have a tendency to roll the bbl out of wooden jaws especially if they are nothing but V shaped.
I also make up lead jaws, casting them to bbl shapes as needed. A simple mould of sheet metal for the bbl to lay in, block the ends where the bbl sits in the cut outs so the lead doesn't spill out. Then fill to half with molten lead.
I use alloyed bullet metal instead of pure lead. The latter will actually extrude under the bbl vise pressure and after tightening the vise bolts, you come back to unscrew the action,,the bbl is already loose in the rig as the lead has extruded from the force.
Two identical casts gives you a set for that bbl.