That is really gorgeous!
Why insul bright? How about fusible fleece? That's what I use for totes. I guess you could put a pocket on the back side of the tote.
Have you thought about window paning the back side? Same sashing just not on point - be a lot easier. Maybe use this on point one for a front flap similar to a messenger bag.
Whatever you decide, I'm sure the finished bag will be gorgeous and well appreciated!