I've been lent a bassinette

, but it's a bit bigger than the mattresses in Big W for $12. I'm going to check at the baby shop on Tuesday after my m/w appointment, but if I can't get one there, the options are: buy a Tetra Ti-tree mattress (which has dimensions slightly bigger than the bassinette), get a custom made foam one from the baby shop near
DH work for about $50 or go to Clark Rubber and see if I can get foam cut to size.
Are the Ti-tree ones any good? I'm concerned its too soft and will mould to baby's shape.
What do you reckon?