I took some balsa wood, about 1/8-inch thick or so, cut it to fit the hopper, painted it flat black, using acrylic paint. While it was wet, I sprinkled model railroad coal on it, and let it dry. If I wanted more heaped up, I took white glue and spread it on, followed by more coal. You can also get some pre-moulded coal loads that may fit. If not, go back to my original idea. You can also work up other loads. For example, I have a farmer that raises sugar beets. These are carried to a beet processing plant in open hoppers. For this load I use a bunch of rice grains, dyed brown or dipped in very diluted brown paint. I use white glue to hold it in place. (Processed sugar is loaded into covered hoppers at the plant, or into boxcars marked "Bulk Sugar" or bagged in boxcars. Of course, you don't see the finished products, except maybe some bags on the loading platform of the plant.) You could also pretend the dyed rice are potatoes.