For regular engraving I also use body putty on a piece of wood. I use a paint stripper gun to get them off.
For deep chiseling I take a 2" piece of steel flat stock and I cleaned it up with a sand blaster (you could file it too, to remove the slag, but i used a sand blaster)
Tin the surface with solder.
Take the piece you are going to work and coat the back with tinner's acid, and clamp it to the plate. heat until the solder melts and let it cool.
Such a "mount" will resist even heavy hammer blows.
SZ