Clear glass is probably the cheapest material. You will need to mix some special glass cleaner (1/3 distilled water, 1/3 alcohol, 1/3 rottenstone powder) to clean and polish the glass surface prior to coating. The advantages to glass are the ready availability, low cost, and the ability to make negatives from clear glass for printing. The downside is fragility and weight. I've also sometimes seen corduroy texturing in plates on alumninum that I suspect is caused by the adhesive for the protective plastic film they apply to aluminum plates.