My favourite Raspberry case is designed by Nathan Jones (many thanks for publishing this), see Raspberry Pi B+ Case with Fan Mounting Holes.
Because I wanted something similar for the Pi4 I have changed Nathan's model to fit the new design.
Dimensions are taken from a purchased Pi4 so no guarantee the case will fit all units.

Revision 1 of 2019-10-17
Now the fan mount holes are moved so the fan centers over the opening. A second top cover with a large opening under the fan has also been added.