Tooling costs for that model would probably be in the "astronomical" range. Maybe a 3-D project for someone? I would imagine that plans exist somewhere.
I did not get into Microtrains, cars or couplers until the late 80s when I became involved with NTRAK. Before that I was helping my dad with his HO layout and was replacing everything in his collection from horn-hook, a menagerie of various die cast dummy couplers and hook & loop types, to Kaydee. All my old N scale was still a mix of very early hook & loop and Rapido. I was shamed by my NTRAK buddies into going with MTs and was better able to afford them by then.
Back in the beginning of MT couplers (1968), MT-5s were 2 pair for $1.50 and MT-3s (factory assembled) were 2 pair for $2.00 so you could buy another whole freight car for the same price which posed a bit of a dilemma for a teenager with not much money. However, I was so impressed by the MTs, I decided to plow right ahead and curb my rolling stock purchases for a while and concentrate on converting the ones I already had. Oddly enough, though, my first 5 - 10 Atlas cars I bought remain with Rapidos. I guess I was trying to preserve the heritage or something. Doug
Micro-Trains has a new car this month just for you: https://www.micro-trains.com/index.php?route=product/product&path=63_64&product_id=4383
Guys, Thanks for this posting about Micro-Trains early cars. I was sure that I had one of the GTW Boxcars. Pulled out my storage trays and there it was GTW 40' boxcar with road number 516798. The conversation also explains why the lettering is a bit fuzzy when compared to later cars. Nice to know that I have a little bit of history in my collection.