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thoughs squishy rubber toys filled with water at the toy store
Along with the more specific squishes everyone else has recommended, I've had great success with a can of Hormel Chili. Open one end (fully remove lid), turn the can upside down, allow gravity to pull the contents out slowly onto a plate, bowl, or pan. Repack the can with the chili and go for take 2. The first take is usually the longest due to the canning process. Subsequent takes get shorter and shorter as more air is in the can when you pack it yourself. But you can get some nice, sloppy sounds that would be a great layer underneath the specifics.