Hmm, coz i use PIS there is a chiller bag that comes with it. It can fit 4 medela btls. SO i bring this bag with me to and fro work everyday. Everyday, after i express my BM i will put the btl into a ziploc bag and store in the common fridge. If you r worried maybe can double layer the bag or get another container. I dun think the fridge to go can tahan till end of the day. If you want to look at my medela cooler bag let me know coz i have another one to spare. Came with my mini electric as a bundle.
Oh, you can get the pigeon storage btls from kiddy palace or major dept stores. And yes, they have the same capacity as medela btls.