Hello! You may want to consider the air-filled baby cot sheet which is made of rubber. It is cooling and comfortable to lay on, and best of all, easy to wash!
Not only did I buy for child, I also buy for my own use. You can get from baby shops like kiddy palace. But they only got 1 or 2 designs. I bought mine from Qoo10, and is cheaper too.
hope this helps!