first of all the overlap is backwards lower should be under the upper and then start sealing. and if you look at the waves if you pulled the liner down and toward the right the liner will sit pretty close to what is ideal to seam. like i showed on the blog getting a piece of pine under the seam makes it a lot easier even if its a battle to get it in or out
not familiar with that product but geocell also makes this which i have used many times including in my pond great stuff. https://www.familyhardware.com/prod...4002&msclkid=06ae758ab5851fb1bff811c464980ecc