I think you could use carpet as a cushion on the sides or top - the purpose
of the cushion is to protect the liner from punctures from rocks, etc. A
bottom drain, I think, will be very useful in the future. Mosquito fish or
koi, they still poop, and you still get leaves and dead plant material
accumulating on the bottom. For overall pond health, you're either going to
vacuum these out or bottom-drain them out eventually. I wish I had the
former!