I'm making my own edging out of screen that I shape into an L cross section about 3x3 inches. I then make a "mold" in the damp ground, sprinkle it with very dry and fine sand the same color as my rock, then spray window sealing urethane, which is a bit flexible over it. Next wearing disposable gloves, I press the screen into it and dump more dirt on to weight it down. I let it sit for a few hours, then pull it out. It's easier to do than to explain.
I can cut the upper edge every few inches to bend it around curves in the liner edge. One edge goes under the rocks or turf, the other down into the pond. So far, that has worked well on the upper pond, I just have to find time to do the lower. The liner is epdm rubber, so sun shouldn't have any affect on it. I won't leave it exposed for very long though. You can see a small amount of the edging just past the burlap wrapped pipe to the bit of duckweed.
I suspect the burlap won't last real long, maybe a year or two, but replacing it is cheap and easy. It's a pretty natural product, so shouldn't be a problem in the pond while it breaks down. Paint flaking off in a few years might be more trouble.