Summary

Top Plate for this case from Amazon, but the Fan Mount is re-centered over the CPU for the Raspberry Pi 3. The difference isn't that big however, and should provide adequate cooling for the Pi 4.

I've included the FreeCAD file if you'd like to edit it.

Print Settings

Rafts: 

Doesn't Matter

Supports:                      

No

Resolution: 

.2mm

Infill:                      

20%

Filament: Comgrow 1.75mm PLA
Grey                    
                        

Notes:                     

Center-distance between mounting-holes in the corners are a bit tight. Perhaps a Horizontal-Offset in Cura would fix it, although I haven't tried it yet

How I Designed This

I originally had the case from Amazon, however it got stepped on, breaking just thin plastic bridging over the GPIO pins. From the plexiglass case, I measured the dimensions with 0.01mm Digital Calipers.

Raspberry Pi 3B OctoPi using the top plate