We use the pottery barn chill box or something like that. I forget the name but it’s a zip around soft lunch box with mesh water bottle holder on the side. It’s fits yumbox and bentgo no problem.

We can fit a yumbox and a camelbak water bottle into a 10" packit lunchbox.

https://smile.amazon.com/gp/product/B01GL7RPI0/ref=oh_aui_search_detailpage?ie=UTF8&psc=1

We use a Skip Hop lunchie (better for younger kids though since they are animal-themed) and a Land's End Classmate. Skip Hop only fits the Yumbox and a granola bar, while Classmate has room for other things, if that matters.

Lands end Classmates is the biggest flat style I've found.

We've had both the garnet hill and an LL bean lunch bag and both fit the yumbox.

Sugar booger lunch box on amazon. Lands end lunchbox. They are rectangular and flat like the yumbox.

We have the Pottery Barn Mackenzie lunchbox and have a Yumbox for inside of it. Not much space for anything else though except an ice pack, fork, napkin, etc.

We had the garnet hill lunch bag for a few years and now we’ve moved on to the regular lands end ones. Both fit the yumbox with no problem.

Just bought a Yumbox original for DD. Unfortunately it’s too big for her pottery barn classic lunch bag. Any suggestions for a lunch bag that would work? Thank you!