Well what works for me can be had at your local Walmart in the Fishing Dept. Look for Thill brand Night Light Bobber LED Light (actual name is Thill Nite Brite I believe). They are available in different colors, and you can turn them on/off. They are small enough diameter to drop into a 30 Cal bore. I have had one in my shootin box for years and still works great, oh and the price is under 5 bucks.
Good Day, Gordy