I avoid soiling the pad and having to use copious tape to cover folds by using a piece of plastic then cardboard on top of pad. Just change out the cardboard as needed.
Pics here:
https://www.backyardchickens.com/articles/pseudo-brooder-heater-plate.67729/
Search google for this one:
Sunbeam XL Heating Pad for Back, Neck, and Shoulder Pain Relief with Auto Shut Off and 6 Heat Settings, Extra Large 12 x 24",
There's only one egress point, that can be a problem.
How many chicks will you have under there?
I'd beware of the smaller mesh openings, maybe even the larger ones, they can push their head up in there and get stuck(speaking from experience).
Got a plan to adjust the height?