okay, so here are the foam rockets that I tried:
There is an opening all the way through the rocket
so at 300psi, it split open like a banana. even though it wasn't a perfect seal above the multi purpose nade, the blast of air still shot through with enough force to cause damage.
I should try it with a regular nade that doesn't send a large volume of gas directly through the middle. Will get a couple more and try a few different things to get it to work. I wonder if just plugging the bottom end with tissue or glue in a foam plug would work.....
One slight issue will be controlling range as my nades are always setup for full power. :/