I used to make all my jumps. One of my favorite fillers for underneath was to get some plastic storage crates, like these crates
turned upside down and filled with fake flowers. You just stick the stems through the holes in the bottom until you have it as thick as you want. You can almost always find fake flowers places like Goodwill. Plus side is they are light, and they stack, even with the flowers in them, if you want to get them out of the way. Old wooden shutters work great as fillers too, if you remove the hardware.