The 18650 battery box is trivial I’ve already done a couple of them through hole, so an SMD version is minor. Do you have a LED in mind (the WS2811b is the controller chip not a led)? Adufruit already has Fritzing parts for their neopixel LEDs and I did a part on a fine pitch 2020 (as opposed to the more normal 5050) led some time back, so there are parts around that can be adapted easily, but we need the package of the exact LED you want to use. Here is a part for the battery holder:
18650 battery holder-SMD.fzpz (11.9 KB)
Peter