Assuming you are placing the coal on a plastic sheet that fits into the hopper, then white glue on the plastic sheet. Cover with coal and let it dry. After adding more coal use white glue cut in half with water and a few drops of dish soap and spray it on.
I actually use windshield washer fluid to dilute the white glue as it already has the detergent and alcohol to eliminate the surface tension and soaks in better.
Pete