I have noticed a lot of the accessories for marine "boats" are a lot cheaper than buying something for a slingshot specifically and the marine stuff is usually waterproof and sturdy. I just order these. They have lots of colors. I am in no way affiliated. Just passing along.
http://www.ebay.com/itm/191752…geName=STRK%3AMEBIDX%3AIT
http://www.ebay.com/itm/191685…geName=STRK%3AMEBIDX%3AIT
These speaker rings will mount under our speaker cover.
Below are pics of the cup insert with LED. Dimensions seem to fit