I used the reflective "bubble wrap" type of heat barrier insulation in cut sheets inside mine on all sides as well as the lid last year. I used silicone caulk to "glue" it in so I could pull it out and replace it if I ever need to. I also installed a livewell timer at the same time.
I had "insulation knowledgeable" people tell me that particular insulation wouldn't work and it would be no better than it was before.
Guess what? I went from fish being dead in 20 minutes or less to fishing all day in August without a single dead fish.
Take that, insulation knowledgeable people.