I am never a fan of the term "must" because people always find a way to do it but not the way you intend. As you note it is not really freestyle either. I did a Christmas theme match where you had Santa's bag in your hand, it had to be dropped down a chimney to activate the stomp pad. It wasn't so big they couldn't throw the bag ,but the opening was small enough that missing really cost you. I would say if you made a prop like the carry on bag check in the airports you could force most shooters to use the bag to activate. Make it such that it really just fit down into it from straight overhead, was boxed in enough where putting your foot there was not a good choice ands throwing it was not likely to be successful. Most shooters will look at the risk/reward and do exactly what you desire them to do.
What are you doing until you drop the bag, are there targets to engage?