Most people size down a half in Rain relative to Simpson.
That being said, I found myself in an identical situation to you, and insoles made a positive difference. Others have had luck with tongue pads from amazon and I imagine they would be better for heel slip as they push the foot down into the shoe rather than lift it up. Worst case? Use both.
Despite being half a size too big, my wholecuts didn't crease awkwardly or suffer any other cosmetic damage. I'm sure they will be fine. If not, well, let me know, I am an 8 UK on the rain
What insole did you use?