Here, a girl is opening a larger, glass container. She is also tilting the box to get a better grip. This is fine as long as the food inside is not something with a sauce or liquid.
The next picture is of a lunch box that was purchased in Hong Kong and reminds me of a Bento Box.
It has separate compartments which hold several different sizes of containers.
The lid snaps onto the rim of the box. I have found that this kind of lid is the easiest for the young children to undo and close as long as the rim is one continuous lip....
instead of a prong lock as in the next picture. These are just too difficult for most 3 & 4 year olds to line up and snap closed.
There are some mass produced boxes on the market now with built in compartments to separate foods. Watching the children opening these, I concluded that they are not any easier than their glass counterparts.
The very best solution I've seen for helping a child with lunchtime independence is in this next picture - each item had been carefully folded in wax paper. The folding was such that it did not come apart in the boy's lunch box yet it was easy for him to get apart. In our neck of the woods, wax paper can be thrown into the city compost so this is also a very 'green' solution and much better than plastic sandwich bags.