You could mount it under one of the side carriers; this has been done on more than one Yuba or Xtracycle build. The closer it is to the main frame, the less of a balance issue it will be, but honestly you'll stop noticing it after a while.
My DayGlo Avenger had just one pannier, on the left side (because of the derailer on the right), so I soon got used to carrying weight on just one side. In your case if you carry stuff in the bags it'll help balance it out, especially if you pack for that.
If you can split the pack it'll fix the balance issue, if you don't want to just live with it.
Since its' only 80mm wide, and assume it's gonna be 100mm with a case around it, then if it'll fit in the triangle via it's other dimensions, you could put it there. Similarly, if the space between seatpost and frame is big enough, it could go there.
If you have no front suspension, you could mount it on a rack over the front wheel, as close as possible to the fork/steerer so it doesn't slow down steering reaction time.
Something else to consider about any permanently mounted pack is weatherproofing. Don't assume it comes weatherproofed.
There is a whole thread about homemade battery cases that should give you ideas.