Nice job and great upgrade. I did something similar, put as thick of insulation as I could fit without redoing the box (1" pink foam board). Sounds like you've already got great results... but one more idea to consider... grab a refrigerator door seal, or use one of the roto seal replacements for Rtic or Yeti, etc. and run that around the lid/seat back edge. Can't see it, and it really seals things up and made a difference for us too.
Totally agree with sonic... it's stupid that these boat companies don't do this from factory. Most throw in a soft cooler, and I get the taking it in and out, but silly not to line the cooler even if it came with a soft bag.