This a variation of the Mama Heating Pad in the Brooder. http://www.backyardchickens.com/t/956958/mama-heating-pad-in-the-brooder-picture-heavy-update
I used a 20"x48" seedling heat mat as my brooder warmer. It requries a thermastat as it can get fairly hot. Alternatively, you could have more ventalation to reduce the heat. Removing the dog carrier bottom might be enough.
Below is the dog carrier bottom
Edited by CalgaryFarmer - 6/14/15 at 7:04am