I nearly gave up on “inside the box” (it took a while to figure out what exactly I was expected to do) but I’m glad I didn’t – that ending sequence was really fun!
I figured it out after about twenty minutes. You need to get the second box of dynamite in the lava and get the box on top of that, so it can launch the box on top of the third box of dynamite, which you can stand on to reach the exit.
To do that: go up front and push the first box of dynamite into the lava (don't dash - that will put it too far, just get it to the right edge of the lava pool). Then once it's in, quickly trigger it, go to the right, and dash into the box to push it left. This is the hard part. You have to get the jump just right so you don't overshoot the box or accidentally push it into the right wall or fall into the lava.
Ideally the dynamite explodes when the box is to its left, which propels the box past the lava and into the hole so it falls next to the second box of dynamite. It's tricky though and takes quite a few tries to do for me. The puzzle after that is much easier.