I always thought that about the bassinets even and they have a special pad you can get to go fit in some bassinets. I would not buy something on your own just because of the safety of it. Products designed for babies are made to reduce the risk of SIDS so I would trust something they make over buying some foam at a fabric store. I know it may not seem like much but it's always better to be safe than sorry. In fact, i used my pack and play downstairs in our house for nap time without any padding and he seemed fine with it...eventhough I know what you mean!!
babies are supposed to sleep on hard mattresses hence why they dont make mattresses very cushioned. Dont buy any padding unless it is specifically specified for the product you are using. I know they have the crib mattress padding but things specifically designed for newborns I dont believe does. Just like they say not to put blankets with a newborn because of the risk of suffication.
Thanks ladies....and your points are exactly why I'm asking. I wasn't sure and didn't want to do anything that would be too risky. I guess when you think about it they would only sleep in it for a certain period of time and the rest of the day when I'm actually up he would be sleeping here and there anyway. I could even put him in his actual crib or swing or something.
I am getting a play pen for my little girl too instead of a bassinet and I agree with what the other ladies had to say about the mattresses. Swings are a great idea and my boys always loved their bouncy seats and swings!
We used one a lot the first several weeks because we just weren't home that much (The air in our apartment didn't work the best and it was a warm spring/summer).
I do have two suggestions though, if the pack n play has a bassinet feature use that, it will help with your back bending over to take him/her in and out all the time and it seems to be more comfortable (at least one ours). The bottom setting you can feel the bars on the bottom a little better. Also, look at different sheets for it, they do make some that are a little thicker/softer. My mother in law found some somewhere that felt like it had a little extra cushion but It still felt just as hard as our son's normal mattress.