If you have any Plastic Suppliers in your area, go buy a 4 by 8 foot sheet of ABS Plastic.
It comes in Various Thickness and most common in Black or white color. (Sometimes in other colors)
.08 to .125 inch thickness is quite good for cases.
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For smaller quantities go to a sign shop, they have much scrap that they love to get rid off. You can almost bet it will be free. I think their choice of material is vinyl but that should make no difference. You can work it like wood and any wood working tool will do, and the color doesn't matter you can always spray paint it.