The TYCO car involved is this one, seen here in the TYCO brown box website, but there are many similar models out there that could be used and certainly from the article Johann converted Athearn cars as well.
as described in a prior article on another of his hopper conversions: “The key to this conversion was that the original three bay covered hoppers in the 1960s were under 4.500 cubic ft capacity and the newest ones today are over 5,000 cubic feet. This means that a very large HO covered hopper is a pretty good stand in for a small covered hopper in OO.”
And these models I think prove that a very large HO four bay hopper will pass in a pinch for a somewhat smaller OO three bay hopper. These are nice cars I enjoy having on the layout, notable also for the nice decal lettering and weathering. The trucks are his HO conversion trucks described in this article.
I have another similar car, still in its four bay state, that given enough time it would be fun to convert into a three bay car. In the article he also proposes converting these cars into two bay hoppers, which look like a very viable conversion as well. It would however be tempting to do a more involved conversion to obtain a closer to full scale car with the post sides as seen on many covered hoppers -- those sides would be relatively easy to make with sheet plastic and shapes. In any case, these cars are one solution to the modern hopper problem, and one that it will be worth exploring.