I second the cooler. Also, if you’re out for a short time, fresh BM can last up to 4hrs out of the fridge, so maybe if it’s a quick outing it would be fine to just store it in a bag or bottle? But for longer time, definitely a cooler bag or insulated container
A cool bag and maybe ice blocks and then the usual breastmilk storage bags
I would say try a cooler bag and a insulated tumbler