Just used standard packaging poly we have at work, don't think it is high density. Then I used expanding foam filler to stick it down and creat some of the rocks.
Sorry for the thousand questions but i read back through and you said you found a clear waterproofing for concrete ponds. Just wanted to find out what the product names is. If you not guessed i've started collecting bits to start a 3D background. Going to try a small one first for my daughters gold fish tank. Then after learning my lessions have a go a bigger scale one.