Is there an appropriate pram liner for a newborn which aids comfort and protects the base of the pram?

Red Nose does not have recommendations for a pram liner, but remember, that it is important that there is nothing loose or soft in baby’s sleep environment or in a pram where baby may go to sleep when out & about.
A pram is not a substitute for a cot (or other regular safe sleep environment)
https://rednose.org.au/article/prams-and-strollers
https://www.productsafety.gov.au/products/babies-kids/kids-equipment/prams-strollers