For sandwiches I like the hard containers. The soft ones always end up with smooshed sandwiches in our world.
My absolute fave containers are the tupperware sandwich keepers.
https://www.amazon.com/dp/B0086KSF6C...a-321601551491 I've always just picked them up at garage sales when I see them.
I also have the standard rubbermaid containers that every store sells and use them often for sandwiches, especially when I make sandwiches out of the bigger bread. They're not as slim as the tupperware ones but sometimes work better depending on what I'm putting in. In fact I have multiple sizes of these and use them for almost everything we pack in our lunches.
https://www.target.com/p/rubbermaid-...B&gclsrc=aw.ds