I have replaced a lot of "discount" liners for customers over the past fifteen years in their koi ponds. The biggest problem with most of those liners were not that they were not tough enough but they did not have the uv rating and/or were exposed to the elements and deteriorated for a variety of reasons ie; uv exposure, rot etc. If you do choose to go that route be sure and protect you liner from the elements by covering both sides. We use old carpet underneath to protect from rocks or roots then at the vary least cover the top with a liner pad then gravel and rock. Be sure to hide all of it because it wont take long for the sun to damage. Typically we will plant a tree on the west side to provide shade for the feature if it is going to be permanent.