As the jack rises there is a tendency for it to move away from the crossmember.
This can be counteracted by means of a home-made “anti-slip” device which is shown on the 3 photos below.
The vertical bolt stops the jack moving forwards and the small piece of wood on top stops the jack moving backwards.
The top and bottom of the device are covered with grippy mouse mats.
To add further safety use long cable ties to temporarily secure the head of the jack and the device to the crossmember.
