Then you need some fresh poop. As much as you can manage. You need to fill your mouth with it.
Pull out the rubber tube ends from the mouth to stretch it, and tie a big knot in the tube. Release the tube and it will retract into the mouth and hold the poop firmly in, a very effective gag. If you wish, cut off the extra ends of tubing.
If you have a very long piece of tubing you can run it behind your head and tie it there too.
On the next page is a demonstration. We wanted to do it in one shot with no cuts, but the poop was unexpectedly big and took us by surprise.
We will try to do better one later.