Making battery packs for an electric motorcycle, what epoxy/potting compound to use?

Weird question, but the context is that I'm making an electric Motorcycle prototype with plans to put it in production in the future, so cost, ease of manufacturing and resilience to the elements are key (and safety duh). My motorcycle will have 16 Nissan leaf Gen 1 battery cells saddling a steel frame, with the cells in groups of 2, 4 groups per side for 16 total cells. These cells currently are completely out in the open, and they need protection from impacts and the elements. After speaking with engineers from Lightning motorcycles, a casing filled with a potting compound or epoxy is a good goal for me. Due to me being a college student with limited tools and money here's my plan: Each group of 2 cells stacked on top of each other will be housed in a 3D printed PETG 3mm thick shell, filled with some kind of epoxy with wires coming out to the inside of the bike. Outside will have a decorative metal grill with a plastic cover. What epoxy should I use? I'm looking for something electrically insulating, weatherproof, thick or viscous when pouring, slightly flexible to allow the batteries to 'breathe' if they need to expand a bit, and doesn't break the bank. I've considered everything I know of from Flex Seal Liquid, to expanding insulation closed cell foam, to electronic potting epoxy. Anyone have any tips/recommendations? Bike photos