Here are the set of tutorials I made:
Backbox: https://pinside.com/pinball/forum/topic/how-to-make-an-led-backbox-for-30-for-modern-sterns
Flashers: https://pinside.com/pinball/forum/topic/how-to-make-flashers-or-under-cabinet-lighting-for-15-for-modern-sterns
Integrate on/off with GI: https://pinside.com/pinball/forum/topic/diy-add-a-gi-circuit-switch-to-an-led-backbox