LOL, congratulations, And yes I AM dead serious. After three decades of homebuilding and being a licensed electrician, I have seen very few (except me, LOL) who ever gave those holes a second thought. One day, a few years back, my local building inspector announces that he wants to see a fully foamed layer surrounding any electrical box that penetrates the outside wall, since testing indicates that all those holes are a major source of leakage. I said great, welcome to the 80s. He looked at me funny, and I told him that a few of us, with half a brain, have been doing that for the last few decades.
The foil tape is important, I would still either cover the whole box in foam, or use foam to seal the back of the sheetrock, against the sides of the box, if you didn't caulk this already. That said, If it was my place, I would order a nice 2' x 4' low profile electric radiant panel and use it to cover the whole mess. It will warm that slab up nicely.