I bought one of the following Blue LED lights for my 120G reef tank for night viewing. Little did I know that the little light would be too bright to be a moon light.

https://www.amazon.com/gp/product/B00X84LRCU/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

I designed this cover to simply snap onto the light and cover some of the LEDs. You can either scale the z axis to make it wider, or do like I did and just print a couple of these and snap them next to each other.