Corriboard Bicycle Basket
Corriboard or corrugated plastic, also known under the tradenames of Cartonplast®, Polyflute, Coroplast, FlutePlast, IntePro, Proplex, Correx, Twinplast, Corriflute or Corflute, is a perfect material for bicycle baskets, in my opinion. It's light, waterproof, durable, flexible, and often forageable from old signs around town.
By popular request, here are instructions on how to make the basket i like to put on my bikes. Adjust as you see fit.
I like to cut extra holes in the basket for a bungee net. Got this corriboard from an old euthanasia campaign sign: