In 10 years of US modelling I have only ever seen the MDC and Walthers cars you mention as Bulkhead flats.
Your only hope is to find older items at meets and on E Bay.
It's a shame that no-one has produced more alternatives.
I have been looking for 60' bulkhead flat cars for a variety of loads. However there seems to be only two on the market: Athearns nee MDC and intermountain. Their retail cost is a bit absurd, so what to do? I have about 20 54' GSC flat cars but no bulkheads. I've wandered around the internet and Walthers has a copy at 7.50. Can anyone help me find cheaper items or come up with plans or references to make my own?
td