I've made my own loads for empty hoppers. Cut a piece of foam board to size and fit it into the car, recessed into the hopper a little. Fill the top with suitable "coal" and glue it in place. You can also do it out of the car so that the loads can be removed if you want. After the foam is cut to size, wrap the piece with painters tape to hold the coal in place and glue it up.
Pretty much my method too, plus here's an additional step I have very successfully used...................
After cutting the foam board to size and fitting into the car, take a cheap plastic grocery sack or plastic Walmart sack or similar and cut the sack apart, so you have two single-thickness sheets of plastic. Remove the foam board or base board from the car, lay a single sheet of plastic sack over the open hopper, and re-insert the foam/base board back into the hopper. Just like lining a cake pan with Saran Wrap or aluminum foil before you put the goods in it. Then pour in your coal or sand or salt or gravel or unobtanium or whatever else you're planning on hauling, mound it up and shape it to suit, and glue it just like you would glue down your track ballast (wet water mist and 50/50 glue mix).
After a day or two of drying time, simply remove the load, discard the plastic sheet of grocery bag, then put the load back into the hopper. Hint: You can speed up the drying time by setting the car in front of a fan. Maybe let it dry overnight while you sleep. Then Waah-Laah - when dry, instant, good-looking load that fits in your hopper like a glove!
Edited: Added this picture of a few of my home-made loads...................
The loads in the Erie Lackawanna hoppers are activated aquarium charcoal, while the gondola and 2-bay hopper are black play sand from the Dollar Store (they look lighter in the picture than they are in person).