I assume you are talking about the new video on layout features:
https://youtu.be/flIrKPVzwoQWhen you create layouts, those red templates (pictured at say 3:40 in the video)
are cells! So just create your custom layout per that video, save it, and when you open the layout, the red templates (cells if you want to use the Lightroom term) will behave exactly as you describe. Be sure before you save the layout to turn the auto crop (button on the Prints tab) off since you want non-cropped pictures in your template/cell locations.
Regards,
Mike