I'm using a 1" metal bezel I found at the craft store, in the UV Resin area of the jewelry section. Take your BFF shopping with you to find something you like.
I've also got some hardcore clear nail polish from the drug store. This stuff is for gel manicures, and it's nice and thick and chip-resistant. Paint it liberally on the back of the bezel, or else the metal will bridge the pins on the NeoPixel BFF and cause it to short out.
Place the bezel on a piece of clear packing tape. If you want, put a little mica powder or other diffusion material down on the tape.
Squeeze a little resin into the bezel. Pop any bubbles with a push pin or by hovering a flame or lighter just above the resin.
My resin came with its own UV flashlight. Shine the light on the resin for 2-3 minutes and it will cure fully. You can also just leave the bezel out in the sun for 30 minutes.
I found it worked best to do the resin in a couple layers, and kept adding it until I got a nice domed effect.
Glue the bezel to the front of the NeoPixel BFF. Do this while it's turned on, so you can center the graphic inside the bezel - it won't be perfectly centered on the BFF but this bezel will cover the edges adequately anyway.
To use the cufflinks, thread the JST connector piece through the first button hole. Then plug in the battery, which will hide inside the sleeve. Thread the JST connector through the second hole to hold the link in place.