So I've got a 9 week old and for the first 8 weeks of his life he was happy to drink room temperature formula. Whilst we make it fresh at home we use ready made when out and about and it's so easy, I just need some cartons and sterilised bottles. However in the last week he's got really fussy and only wants his formula warm. What do you think is the best way to heat pre made formula on the go? I can't find a portable bottle warmer with good reviews, and I don't really want to cart around everything I need to make fresh formula if I don't have to.