What material to use for rolling suitcase/backpack?

Long story short, I need to build a rolling suitcase/backpack that will have dc motors in the wheels. The motors are not very strong(2 motors both powered at 6V and 3rd wheel without motor for stability) and here are the actual specs:
What material would be light enough to keep the rolling suitcase/backpack moving at a reasonable speed with the 2 motors I have?

Why not connect a couple to a shoe box, see how much weight it can move around as a starting point.