I haven't done my belly band yet, but what I've used everywhere else - windows
and through-hull fittings is: Pro-Flex. A tube for a caulking gun and it comes in white, black or beige. Pro-Flex seems to be what everyone around here (Pacific NW) uses. It works and stays flexible. Follow the instructions. Just don't use any silicone-type material !!
Good luck with your leak fix.