Here are files for Pi 3 and 4 enclosures should you wish to 3D print one. The ultrasonic sensor mounts on the underside of the cover through the cat's eyes using the 2 standoffs and 2 M2x8 screws. The Pi mounts to the body with 4 M2.5x8 screws. The body mounts to the wall via 2 #6 or #8 screws/anchors.
If you're having trouble getting the Pi in place, try rotating it into position as shown here:
