Hi, is it ok to use waterproof mattress protectors lined on the inside of a fitted cot sheet and what material would be the safest to use?

Red Nose recommends that baby be placed on a mattress that is firm, clean, well-fitting & flat.
It is important that the mattress protector does not compromise the firmness of the mattress, not necessarily the fabric of the protector.
.....always make sure that the waterproof mattress protector is strong and a tight fit.
Never put soft bedding under the bottom sheet, such as a sheepskin, as this makes the sleeping surface too soft.
https://rednose.org.au/article/what-is-a-safe-mattress