Dear Adam;
I used the Bachmann O/S scale coaling tower on my, now dismantled, garden railway as a rock bin. I plugged the door to the hoist shaft with siding material. I added breather and exhaust pipes for the "virtual" hoist engine. I removed every other step from the O scale ladder to the front platform, and fabricated new raised railings. I also removed the platform and stairs to the hoist shaft door. Finally, I mounted the whole structure on some 2x4 lumber sections to raise it up and give it a low center of gravity.
You could certainly do the same with a double rock bin. the photographs below should give you some ideas. By the way, my coaling tower dates back to when I was about 12 years old, which makes it 49 years old by now!



Have fun,
David Meashey