I’ve never done gleying, so I don’t know any tips and tricks for that. We had our pond sealed with bentonite clay and that worked like a charm. Bentonite isn’t expensive and if you have some tools around you can probably DIY. Saves some time over the gleying option and it is more natural than epdm.
You can checkout
https://lonestarbarite.com/bentonite-clay-pond-liner-ponds-and-lakes/ for more info on sealing with clay.
