I designed this piece in FreeCAD. I wanted to mount a Noctua 40mm fans for intake and exhaust, the upper port is non-standard so I made a simple adapter to attach a standard 40mm fan. The lower exhaust port did not extend out to the back plate. The inner surface is a tight fit to the original mounting homes so I just put some double-sided tape on the bottom and push it in tight. No screws needed on that side.  So far, there has been no slippage. I used PETG for the prints.