A box full of dead nutrias works well, but gets messy
Go to the grocery and buy up all the boxes of Jello... and mix 10 to 1 w/ water.
Seriously, you can buy it on the internet. I don't remember who had it (midway maybe?) but I've seen it at one of the bigger gun websites before.
Uh, a 17-1/2 x 11-1/2" x 5" gel block from Midway is $170! We could buy a lot of jello for that.