The original battery capacity is around 60mah, I changed it to 220mah so you can play music longer without charging.

There are two variants. The first one is universal and should fit all BT receivers that shape but you may need to figure out how to keep the board in place so it would not bend when you click the button and second version is better, it should hold the board inside well but may not fit all the board variants so it's on you what you choose and what will fit you the best( with second variant you need to have all connecting surfaces flat so you will need to cut that small things off(you will see)

(use 0.15mm layer height)

Battery 3.7V Lipo cells 501240 220mah (https://www.aliexpress.com/item/1005005880402193.html?spm=a2g0o.order_list.order_list_main.95.5a521802OpG7OP)