A decent sized jack with a large head on it has never been a problem for me

(resting on the gearbox end of the sump with a block of wood in between.

)

If you're really worried about movement or if your jack is small enough that the engine is balanced a bit precariously then I guess you could use a sturdy length of wood

(at least a couple of feet or so long) under the same part of the sump running front to rear supported at each end with an axlestand/jack/pile of bricks or whatever is to hand.
