I have just purchased a second hand cot off of Ebay and want to buy a new mattress (obvs). I know that foam isn't recommended as it could be a factor in cot death. Beyond that I have seen sprung, pocket sprung, wool, coir, anti-allergy and combinations of all of these together. Plus waterproof covers. HELP! Can anyone tell me what they think a good mattress is made from, or what the differences are?