I designed this bottle stopper to hold the batteries and switch of these small LED holiday lights I got online. It turns old bottles into holiday decorations. I made 2 for the bottles I had. One has a 23mm opening and the other is 18.8mm, I could do other sizes if there's a demand. Above the round part they are almost the same. There is also a cap to hide the hole in the top.

here is the light strings I used.   https://www.amazon.com/gp/product/B077N8J77P/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1