So far, in my searches, there aren't extended-life batteries that have a separate housing backing for the phone. There aren't even any attachable battery juice packs (as they are often called) that plug in and snap on outside the phone like a phone case. I agree, the great advantage of a removable battery is the possibility of an extended battery being made, as you state. I also like the thicker grip as well, and I'm willing to sacrifice a little in size for better battery life.
In the past, Mugen Power has been the most proficient at making these batteries, but they also are per demand. So unless they get a demand for this model, they may never make one. I messaged them and haven't heard back from them. In the past, they were very quick to reply, so they may just be a lot bigger and more in demand than to answer emails from consumers these days.
Hopefully something will come later this year from somewhere, and at least hopefully MS will continue to optimize the OS to help with battery life.