The principle behind the heading pad system is direct contact heat. That means the chicks need to be able to make contact with the heating pad with their backs. I can't really see if the "lean-to" setup accomplishes this.
A better way is to cut a scrap of steel fencing and bungie the heating pad to the fencing frame. Enclose the entire thing in a pillow case so the chicks can't crawl between the steel fencing frame and the heating pad and suffocate. Bend the frame into a "U" shape, so the chicks will fit comfortably under it.
As the chicks grow, doubling in size every week, I adjust the frame so it's higher to accommodate larger chicks and still allow them to make direct contact with the heating pad. Also, you will be lowering the heat each week as the chicks feather out since they will require less heat with feathers to prevent loss of body heat.
In just one more week, if they haven't already, your chicks will not use the heating pad during the day. By age five weeks, they won't use the heating pad at all unless it's freezing at night. At that point, they will be able to live in a coop.