Cardboard is the perfect medium for making a splint in an emergency. Any piece that still has integrity will work, including lighter weight cardboard that has not been excessively crushed or folded.
The cardboard should be long enough so that it comes at least halfway up to the calf. It should also be broad enough so that it can be folded around the foot in sections with a left panel, center panel, and right panel.
Using the edge of a desk or the corner of a wall as a guide, make two vertical creases in the cardboard. You will want to ensure that foot and calf rest snugly in the center panel, while the side panels can wrap around the foot and be more or less square.