Mark:
I wouldn't waste my time with only 6" of clay. I'm of the opinion that if I'm going to have the equipment on site to
properly put a clay liner in my pond, I want no less than an 18" thick layer all the way to top, and really, really would like 24" thick layer of
properly placed and compacted clay. Not just the bottom, all the way up the sides to a point that is higher than the water level will be. I put properly in bold because there's a right way and many wrong ways to put clay in a pond to seal it.
Now on to liners. Think of a liner as the ONLY thing stopping the water from leaking out from your pond. With that in mind, would you rather go thin or thick? Keep in mind, the liner will be there for as long as you own the pond, and have water in it. Any hole or seam that is not glued together will let water escape.
https://www.btlliners.com/ FWIW, I called around and got quotes for close to $30K to line my pond when I renovated it. I didn't do either, and now am looking at $5K+ for the well plus monthly electric costs to keep my pond full with a well.