Any heating pad will work just fine as long as the thing won't shut itself off after a couple hours. Right now I have an old fashioned three-speed heating pad set up in the run so my three-week old chicks can warm themselves on it since it's a chilly morning.
They have a MHP cave set up inside the coop with a heating pad that has six settings. Currently, they are down to the coolest setting at night since they are getting close to weaning off heat.
Read through the MHP thread by
@Blooie.
https://www.backyardchickens.com/th...d-in-the-brooder-picture-heavy-update.956958/ It has all kinds of suggestions and warnings as we became users of this method. Some folks, myself included, offered alternative iterations and shared experiences where we modified things due to a chick getting stuck between the heating pad and frame.
As for selecting the right heating pad to buy, it depends on how many chicks you will be brooding as the pads come in extra large and some models even have a control that resets itself after a power outage. This beats having to get up in the middle of a below freezing night to go outside in your jammies to start up the pad during a storm that knocked out the power.
@Blooie had that experience and tells about how the pad retained enough heat to keep the chicks from freezing.