Look for "ore car" or "ore jenny" plans. They are the only steel single hopper cars I know of. You can probably find plans for a EBT twin or triple steel hopper for 3 ft narrow gauge.Over the years I think MR, RMC and NG&SLG have all had plans for the EBT cars.
Dave H. Painted side goes up. My website : wnbranch.com