get some glowing party ballons from ebay and cut out the LED and battery then with a glue gun glue into the void on the inside of the foster slugs and your good to go!
i havent seen any videos of these on youtube so i guess no one knows about it yet i got the idea because tracer slugs are illegay where i live.
i have done this and it does work and with the glow coming from the back only the shooter can see them glow