Pond liner is superior over anything else, if you can afford it. Other tipes of lining like shower curtains work great but you have to replace them eventually. Pond liner is indestructible. It lasts forever
Welcome to the forum. Could you post pictures of your enclosure? It could help everyone around here to help you make any improvements still needed. You sound like a caring person and we all want to help you. You've already received wonderful advice but pictures say a thousand words.