SUF shared his voltage reference project in the forum:
To achieve the things above I designed a small battery good indicator circuit with a zener an opamp two LEDs and a few resistors. If you switch it on and the battery fall below 8V the red LED will lit instead of the green.
I wanted to have a SOB style case. I designed a new one with a trick on it. I used a third acrylic plate what has a hole inside. The hole can accommodate the battery and the battery clip. This setup keeps the battery in place.