The problem you have here is there is only an STL file and while you can import that into a lot of CAD programs it doesn’t have any of the original model info, it is a single body with no model history that can be modified, which makes revisions difficult but not impossible. I did down load it and many CAD functions won’t work on this with the program I use. It might be easier to redraw it from scratch with the mods you want. If you send me the dimensions of the fans and the card I can model it for you. It may take a few days, other commitments.
I converted it to a STP and a Parasolid file and neither of them have any “bodies”. I can’t even open a sketch on them directly to make a mod or even get a dimension. I can make an overlay sketch but I will still need some dimension. Diameter of the holes, bolt hole spacing etc.
If my cohort here (Loosenut) has difficulty with it or dimensions not available, I may be able to help. Can work with the stl file. It won’t be as pretty/precise as a cad file but will work.
Just a suggestion but this is not a massively complex model. It would make a good project to teach yourself with. In my experience having something you want makes teaching your self easier. Sometimes starting from scratch is easer.
I took the easy way out and made a new back plane over the existing one. See pic. I ran it though a slicer and it seems to be OK as shown. I need the dimensions for the new fan holes, mounting holes, OD dimensions of fans and do you want them spaced evenly?
OK that wouldn’t work, the underlying “ghost” model can’t be modified. I redrew the model from scratch. I still need all the dimension for the fans you are going to use to finish it.