what about putting a blanket in the dryer and warming it up and then putting it on the cot just before you put bubba in!
I would avoid those heat packs that you click to activate as i used one and they can get hot spots on them and are a pain to reboil after each use.
You can get the disposable heatpacks from the reject shop. A pack of two is like $2 so cheap but once agin watch the heat as they do get hot!
HTH
odette