Insulating Behind Completed Kitchen Cabinets
Without a do-over, it seems like the best you could do is address the convective loss (the flow of heat out, or cold in, via those openings).
I'm curious how much improvement you'd have by just taking some unfaced fiberglass insulation and stuffing it into the opening, maybe enough to partially fill the area just up from the opening, but the less compressed, the better.
The gaps look too big to try closed cell spray foam, otherwise that would be my next suggestion.