Have you checked the temperature on the cool side of the brooder box? It could be the reflection, but it looks like the cool side is still getting quite a bit of heat. They'll need a cool area as much as a warm area so they can regulate their body temp. If the cool side is too warm, you can...
The rentacoop heat plate should have a post on each corner. Use @rosemarythyme advice and keep the back end low so they can physically touch the plate and the front up a couple of inches so they can find their own sweet spot. I know the new place isn't set up yet, but do you have any pics of...