I researched the Mama Heating Pad idea because it seemed nice to do and was more affordable to me at the time to make one than a broody plate. Good advice on it on BYC, need to have different heat settings, for one, and this particular model had a very good safety rating. I have taken off the outer cover and wrapped it with towels and old pillow cases. There is probably updated current advice on what to get. I have had it on Low but put it up to Medium last night when I went out to investigate the autodoor and close the manual door.
In May 2020 I paid $19.99 for it. As
@Ponypoor would say: Holy Dinah it's $51.92 at Walmart right now. That's scandalous.
Sunbeam King-Size Moist/Dry Heating Pad with UltraHeat Technology and 3 Heat Settings (732-500)