Did you purchase yours second hand? From what I’ve gathered, it should come with one. Especially if bought within the last 6 months.
You may be able to see if someone on the forum may be selling theirs.
Or better yet, if it was second hand, see if the seller perhaps has it somewhere. As for making your own, I suppose you could, and possibly make them mountable anywhere. If it’s helps, I can take a photo of my board for you to etch your own. There’s probability a schematic on GitHub somewhere.
That should get you the pin outs at least.
Honestly, it isn’t the most eloquently designed add on. I have modified the back shell of one of my cases fo have the additional keys built in. You could probably do that yourself.
Did it come with the lego brick style back case for the gameshell? The light key mounts onto that with two little clips. It should have been on one of the runners included in the box.
If it wasn’t, give customer support an email: email@example.com
Failing that, you can 3D print your own:
Possibly one of the most eloquent integrations of the lightkey is this one:
I personally just run my gameshell without the light key attachment.