I found, after several design iterations, that the pad is best under the rack.
If on top, make sure the rack has small holes or they can push their heads up thru the rack and get stuck.
Also gave up on wrapping the pad in anything, can just create gaps where they can crawl in and get stuck.
I put a piece if plastic between rack and the piece of cardboard on top to keep pad clean. Cardboard can be scraped off or changed out.
Here's mine:
https://www.backyardchickens.com/articles/pseudo-brooder-heater-plate.67729/
If on top, make sure the rack has small holes or they can push their heads up thru the rack and get stuck.
Also gave up on wrapping the pad in anything, can just create gaps where they can crawl in and get stuck.
I put a piece if plastic between rack and the piece of cardboard on top to keep pad clean. Cardboard can be scraped off or changed out.
Here's mine:
https://www.backyardchickens.com/articles/pseudo-brooder-heater-plate.67729/