You could try ruck touch, Iain.

Split the group and play standard touch and pass before conditioning the tackle area. Tackler stays in and can try to "jackle" the ball which means one attacker has to ruck. Condition attackers to pick and drive/attack the fringe thus forcing defenders to post guards. Once that happens defence has to work inside to out and any over-commitment will leave them short out wide.

You can make the defence play short-handed to increase the pressure on their reading of the situation and communication.