The walls of my pond are very rough clay with lots of jagged bumps from
being chiseled out. I was originally going to put underlay around the sides
and bottom, but the walls are too steep. I could use a few long spikes to
hold it in place, but I'm concerned the frost will pop them out into the
liner. The clay is too hard to chisel smooth, it will just crack and create
more bumps.

I'm not sure this really matters though, since the old hole was smooth as
glass when I took the liner out. Granted I dug it earlier in the spring when
the ground was softer, but I can assure you I didn't spend all that time
polishing. So perhaps time and the weight of the water will be all I need. I
did not really encounter any tree roots when enlarging the hole, which was
surprising.

Any ideas on smoothing/covering if required at all.