For a hopper you could use a 100-ton coal car, the one you dropped on the floor and broke, remember? Just use a bit of trial and error, starting out small, and use a Dremel to grind a hole at one end. Maybe use a piece of card stock or thing plastic as a baffle to adjust the flow. Similar to the chute cover on a grain trailer. Use a small piece of plastic stock for guides. And as they say in France, VIOLA!! or is it Voila? Not being French, who cares? How about BAM!!! It's done.
But thinking a bit, maybe a high-sided gon would work better. Or a cheap box car, more grinding required here though. The bays of a coal car wouldn't let the ballast empty smoothly.
The car would have to be mounted to allow the ballast to head downward to the front of the "ballast regulator"/car.
Bob