I have an old case that I put a 386 in, but I did not have the original drive bay covers and it was an unusual design, so I did not have any compatible ones either. I wanted to learn FreeCAD anyway, so I used this simple design to do so.
I have included the step files if you want to make adjustments.
The photo shows two of the covers installed in the case and the OEM cover on top of the case. Please pardon all the yellow.
The author marked this model as their own original creation.