Cut your cardboard into strips where the edge will be just below the top of your tin so the lid will fit back on.
Cut it long. It takes a lot more than you think.
Roll the strip up, stick it in the tin, let it go.
If you got too many gaps, simply wrap some more cardboard around the outside your strip and put it back in. You want it pretty full, but not completely packed.
Here's about where you wanna be: