I'm throwing around a couple of ideas....
Simplest would be to bend up a bigger hopper that would just slide down into the current hopper. It could be made out of anything, like cardboard, plastic, sheet metal...and you could just pull it out and put it away. The current hopper would secure the bigger hopper in place.
I also thought of bending up a new one out of sheet metal with the same bottom size as the current, using the same mounting screws.
Another would be to sacrifice a 5GAL water jug by cutting the bottom out of it, and using it as a hopper. Because of the weight of 5gal of grain, I'd need to secure a sturdy mount on top of the table to hold the bottle. Could continue to use the current hopper in combination with the bottle.
Still not sure, but I think I'll end up with the simplest...fashion a bigger, slide in hopper that can be easily slid out and put away.