Mix the hot water, glue and cornstarch in a bowl using a fork. Whisk until the mixture is smooth and even and without lumps or thick spots, like heavy cream.
Fill the cereal bowl about halfway with the glue solution.
Pour cereal into the bowl to fill it. Gently press the top of the mass of cereal to push it down into the glue solution and let the liquid fill in the spaces between the pieces.
Place a spoon in the bowl. Leave it there if you want the finished cereal prop to have a spoon in it. If you need a prop spoon with fake cereal it in, take out a spoonful of cereal and glue solution. Prop the spoon on a piece of balled-up aluminum foil to keep the cereal from sliding out of the spoon while it dries.
Let the glue dry overnight in a well-ventilated, low humidity area.
Tip the bowl-upside-down and gently shake it to let any remaining, loose pieces of cereal fall off the top. Use undiluted craft glue to glue these pieces of cereal back in place if you think the bowl needs them.