Well, added my 21st cell today and below are some pics. I did not mess with the existing battery box. Also further down is some measurements and mod I did for the fuel gauge (even if you don't plan to do this mod, the info does show how the current XM3500 is setup - I think it was set for the original 19cell as I would prefer not to hit the RED recharge point with the cells this low).
Looking more at the battery and considering swelling, I decided two things; not to mess with the existing battery compartment since is has them held tightly and then to mount the additional cell so it is "compressed". I also moved the circuit breaker so I can access it through the existing hole under the seat yet making the wires and connections much better.
In the second picture you can see the Paktrakr I added and my BMS (the BMS unplugs when I ride but the Paktrakr is permanently mounted). I will post another topic on how much better the bike performs once the weather improves (it is cold and rainy).