i don't do a lot of "crafting" things... basically just drawing and painting with the kids every now and then, so i guess ididn't really understand...
posterboard to make the shape of the cone and wire to shape the rim and then i use the glue to... idk how am i supposed to use the watery glue... they are 3 and four they could probably just wear it with the poster inside...
If you used the glue, whatever you choose is fine but, all you'd need to do is make a circle with the board. Cut a slice to the center.(radius) Then overlap to make a cone. Staple or tape the cone to keep it's shape. Place hat over and saturate. The glue will be absorbed by the cloth basically making it like a laundry starch on steroids. It will dry, inside the cloth, becoming stiff overnight. This will allow you to remove the cone and use as originally intended.
EDIT maybe a quick spray of food release(Pam not butter flavoured) or wrap the cone in plastic wrap, to make it so that the hat doesn't stick to the form.