As you are finding the covers are hard to find especially with all the cover doors and they are not very inexpensive.
Here is a reprint of an the OGR topic I shared on making an easy hopper car cover. The center fill type cover is easier to make than the 12 hatches of older covered hoppers. Others are included. They are easy and quick to make.
I make an ALCOA covered hopper car from an open hopper car by making a cover from sheet balsa wood and adding an ALCOA sticker.
Here is a ORMET's alumina covered hopper car.
Here is my new center fill hopper car. I purchased a Reynolds Aluminum hopper car body without the 12 door Top at a train show.
Center fill is more modern than the 12 individual doors and quicker to fill.
The first picture shows construction with grating being made from leftover fiber window screen material which makes an easy source of grating for model trains. Roof is made from hard 1/16 in balsa wood.
The Second picture shows the finished painted roof.
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