The idea of sealing the bags of expressed milk sounds worth looking into.
I've been carrying the bags from Breastfeeding Support Network for several
years.  The moms love them.  Sealing is so simple because the tie is built
into the bag, reducing the chance for spilling and ensuring a good seal every
time.  And they are reasonably priced, compared to other major brands.

Kathy (who holds no stock in BSN but loves their product)