I could not find any case for old raspberry pi 1 rev B. with mounts for Ender 3, so I mixed two nice projects to produce one:
The design of the case is quite nice, it was already was a remix made for pi 1 rev. B, with mounts for a Prusa printer:
Another project proposed a case for Raspberry pi 3, that we can snap on the Ender 3 rails without screws:
Place the mount system from the second one on the first one, and you get this.