I use a 12" x 4" x 3/4" plank with a notch in the end to fit the engine. I screw the engine to the plank. I then "C" clamp the plank to a saw horse with two clamps. To the plank I use #64 rubber bands to hold the tank and slip a 12" wire rod with clevis attached to the throttle under a rubber band for a friction hold on the throttle.

Since one of my OS 70II Surpasses once took a saw horse down the driveway I also tie the sawhorse to a tent stake in the yard.