In the first iteration, don’t allow the throwers to actually throw, but they can give feedback if the option was open. After everyone has had a go, if the option seems very open, throwers may hit the cutters.
To start, the buffers may be quite large, as participants improve and shut down options, prompt them to slightly decrease their buffers.
Number one priority is stopping the open under, even if the defender is buffering correctly, they need to not get beaten here. Tell them to increase their buffer for this space if necessary.
This can be made a scoring game for defence as follows:
3 points - if defenders get a block, force a turnover or stall out.
2 points - if cutters catch it in the break side
1 point - if cutters catch it in the deep space
0 points - if cutters catch it in the open under space