A Savage nut wrench (grooved nuts, either round or square grooves) can be improvised with a large open end wrench that just fits over the nut, and two 1/8" diameter pieces of brass rod. Brazing rod, etc. The rods go in opposite grooves, and will engage the wrench, allowing torque to be applied to the nut.
There is a member who has made barrel vises; don't recall who, though. I made and sold a few, but the material cost, machining time and shipping made the price higher than I was aiming for.