I purchased a moses basket which came with a bad mattress (approximate size 70X30cm). It was completely covered with PVC plastic, so a most uninviting, non-breathable and cold material for a new-born baby. It also lacked firmness so I decided to replace it.
After long research I purchased the Ventiflow mattress and it's very very good:
+firm enough to support a new born baby's back correctly
+breathable fabric surface, lightly quilted
+plastic covering only on the underside for any accidental mess
+very decent price
My baby seems to be comfortable sleeping on this mattress and so far I have been happy with the quality!