Nothing wrong with making your own deadboard but am somewhat confused as to the concern for using a 1/4 inch beadboard panel. I am not aware of any of the commercial door manufacturers that sell flat panel doors that use anything other than 1/4 inch panels. Your door strength comes from well built rails and stiles. If you are painting them you can buy 1/4 inch MDF beadboard in 4x8 sheets The rattle concern can be handled with space balls or any other similiar material. A drop of glue on each side would also alleviate this concern. For the MDF sheets I would check with lumber yards rather than the big box stores. Good luck with your journey

Brian