This will fill the gap between the SD card slot and the outer case on the Ender-3 so that there will be no chance of slipping the SD card into the case.
May need to rotate the model in Cura like I've shown in the pic.
Requires an M3x10mm
This appears to no longer be required for the newer machines.
I used 20% infill but it probably doesn't need any as it won't be taking much force.
If you've having trouble with the part sticking to the bed then just add a brim.