There are probably as many different ways to make a coal load as there are open-top hoppers, but here’s a method I’ve just pioneered.
I use Harvey’s High Temperature Furnace Cement as a modeling putty. It is intended as a filler for cast-iron stoves, and I had picked some up in order to test its utility as a sculpting putty on a cast-metal model I was working on. Tangential point is that it did work very well in that role (good adhesion to the metal, reasonably long workable time, decent initial consistency); the only negative is that it has a little bit of roughness to it.
I first cut out a rectangle of cheap cardboard (use a cereal box or similar packaging) to fit into the hopper’s opening. Then, using a plastic spoon as a trowel, I scoop out (using the back side of the spoon!) a small amount of the putty and get the top surface of the cardboard covered. Add enough to make you coal heaps as you see fit, then spend some time shaping it with the back side of the spoon. Then, once the shape is roughed in, sprinkle on some ballast. I used a ballast of the right texture, intending to paint it afterwards, though perhaps one could use real coal loads if you don’t mind adding a black wash aftewards to hide the light grey of the putty.
The cardboard will warp with the moisture, but this is fine; over time, the weight of the putty will settle it back down flat. Use the edge of the cardboard rectangle as a guide to scrape putty off the sides, to make sure it conforms to the hopper’s shape. The layer of ballast on top of the putty should be pressed into place, and you can adjust the shape of your heaps by pushing it around with your fingers; the dry ballast won’t stick to you. After it is dry (in a few hours), you can then paint it however you want.
The load that is produced had a bit of weight to it, so it will sit loose in the hopper. Plus, a little extra weight in an N-scale hopper is always a bonus! I had previously tried making loads by carving balsa block