Do not put a towel on top, there are air holes that need fresh air circulation.Is it super super concerning for it to go up and down a bit? I don’t really have another good spot for it. I forgot to put a towel under it so I can do that now. Should I put a small towel over it too? Or not?
It might not be too bad if it goes up and down a little. If it stays too hot, they could hatch early, if it stays too cold, they could take longer. If they hatch several days before or after 21 days, then there can be defects.