BorgCube
I wish I had more details, but I have bought N scale slag car kits from Peachtree Shops in Laurel, MD. The owner is a big steel mill fanatic in HO but has several products made for N. A slag car kit is one of them.... I will try to find more info and post. They do have a web site - peachtreeshops.com I think....
All the best!
Bill Deuster