You could use a closed cell pad and synthetic sleeping bag, or reflectix homemade pad. But they make some dry bags (and even ones that taper to fit in the end of the kayak) that will 100% keep things dry even if dunked
Dry bags are great. I keep my clothes, bedding and things I want dry in them. Important things get double bagged. I still get things wet. Sometimes I'm a klutz. When I was in Alabama my fishing partner started calling me splash.