flowerbug
Garden Master
Check with an appliance store. They often have huge boxes and may be willing to let you have them.
also check auto body repair shops. we've given our local auto body place so much business the past 15 years that they're always glad to see us. we figured at least a few cars worth between the two of us and three different cars.
with cardboard you definitely want as few cracks as possible or use a few layers and overlap by about a foot.
craft paper will let some light through, but newspapers and smaller chunks of cardboard can help there. also mulch over the craft paper will cut down on light going through.
in all cases it is only temporary anyways depending upon how much rains you get there and what walks on it. for me i'm glad if i can get one season out of it, but sometimes i can get two if it is in a lower traffic area and the deer don't manage to shred it when they walk on it.