I had a quick look on the 'net and found a few places that have bassinette mattress protectors. If you Google "Bassinette Mattress Protectors" you should get some results. One brand I found is Playgro and it is waterproof, hypo-allergenic, etc., and at $5.50 each you could buy a couple to be sure the mattress is always protected. Not sure how it is secured (as I said previously, our protector has two elastic straps that go under the mattress) but the size seems appropriate - 50x40cm. You only need it to go under most of the baby, not all of it - they manage to get their messes in a lot of places but we find it is mostly contained in a relatively small area - "bonnet to booties" explosions are pretty rare!