My sons have PBK lunch bags. Here's an ebay link that shows interior pictures of the exact model they have (Varsity): http://tinyurl.com/4k6a9t6
My older son has been using that lunch bag along with a Sistema water bottle and Sistema lunch cube from the Container Store for 2 years and they have worked GREAT. I'm still using the original ones I bought and really like using them. My younger son has also liked using them this year, and I pack our picnic lunches for our family with these containers too. Here's are the links:
Lunch Cube: http://tinyurl.com/4hlw8fj I've written about this cube before on here but I LOVE how easy it is to make lunches with these. Usually, I put a sandwich in the large area (fits perfectly), fruit in another side, and veggies in another side. But it also works to have crackers or a dry ingredient in one of those sides too--they don't mix together.
Hmmm....I can't find the water bottle on the Container Store website but here's a link to an item on ebay to show you what it looks like: http://tinyurl.com/4l9uf4y The water bottle comes in the same colors as the lunch cube and in two different sizes. Both sizes fit into the PBK lunch box and they also are skinny enough to fit into the elastic strap inside the lunch box as well as into the small water bottle holder on the outside of their PBK backpacks.
With the lunch cube in there, the water bottle fits perfectly so I don't put it inside the elastic strap--no need. The bottle doesn't leak if it's closed properly. The only thing to be aware of is that you can't fill it completely full or else some will squirt out when your kid goes to take the first sip. If you leave a little empty space at the top, then it's fine. It's not stainless, of course, but it is BPA free.
ETA that I only give my kids water in it, so I don't know how well soy milk would work in it. I do stick the bottles and caps in the dishwasher, so I assume that the milk should clean out OK. Of course, you'll want to include an ice pack (which the PBK bag does have a place for).
One more thing--their lunch cube fits perfectly into their PBK bags but it doesn't work well with the LL Bean critter lunch boxes, which are more narrow. I personally have had much better experience with the PBK lunch bags than the LL Bean ones. In fact, I have two Critter lunch boxes that I got rid of because the zippers became hard to use after only a year of use. Once my preschool son spilled milk in his bag (with different containers than the CS ones), and after that the zipper became nearly impossible to use. In contrast, I have washed his PBK lunch bag a couple of times in the washer and it's still going strong after two years and he will use it again next year. It's also very nice to be able to clip the lunch bag easily onto the outside of the backpack. (He's been using his PBK backpack for three years and will also use it again next year.)