You will definitely need something between the fleece and the coroplast. The purpose of fleece is to wick the urine down to an absorbent layer under the fleece. If there's no absorbent layer, the fleece just stays wet and the pigs' feet get irritated. You can use mattress pads, u-haul blankets, wood pellets (the kind used in horse stalls), and other things. There are several threads on fleece here and over at www.guineapigcages.com
You'll have to wash the fleece several times in hot water and strong detergent before you put it in the cage. That will make it wick properly.
You can get it at any fabric store, or online.