needs to be impervious to water, Can be in pieces as long as they are roughly 3 foot squared or 4 foot squared.
Bits of membrane from a damp proofing course would be ideal. This is for a project so as long as there is enough to cut large circles out of it with a rough diameter of 3 foot or so, that would be great. Might just get away with slightly smaller or using gaffa tape to make the size up.