Having a look at the video (is this it?
https://www.youtube.com/watch?v=cnjkWS35Rvc) there is definitely sub-optimal room in there for an LM.
Click to make biggerFrom what I can see in it's factory configuration there appears to be not enough length available, an LM may not fit between what appears to be part of the chassis on the right and the iPhone speaker on the left. If it does and you use an adapter harness to keep the motherboard then vertical space above the motherboard might be an issue.
The approximate available length could be worked out be from the video capture by using the length of the iPhone speaker and doing some math but the available height can't be measured from the capture and would need to be measured physically. I don't recall what I did with the couple of iPhone speakers I have so I can't get the measurements for you at the moment.
I don't envy this given it looks like a whole lot of wires but it looks to be a hardwire job disposing of the motherboard. If length is still an issue then perhaps the chassis protrusions on the right can be cut off, or the iPhone speaker can be made shorter (I've not had much success doing that but it is possible).
or, you can can send the screen capture to Ring and ask
"are you working on that 21 pin LM, or do you expect me to use a harness here?" and see what response you get.
- Tim