Plattens are almost always steel. Meehanite is often used. Aluminum will have good heat transfer but is soft and easily damaged, and will not wear well. Tooltech is teflon impregnated fiberglass,which is very soft and can be considered a disposable liner. Your press must be very very low pressure processing. A thin sheet of stainless steel is one option for better durability, but Tooltech may be fine for your application if your process is a gentle one.