While it may be lighter, cardboard is a tad weaker than fiberglass.
Submitter MarchNero says: Whenever I'd go biking in jeans, my pant leg would always get caught in between the gear and the chain. To fix it, I simply taped a cardboard circle to the pedal. Works like a charm.
From the submitter:
A do it yourself computer tower fan. The fan and Power Supply went out on our computer tower so my husband came up with this..and it works.
It'll last for 10 years... or until it rains next. Whichever comes first.