Melamine is the most common DIY rack material out there I believe. It is typically white and can be purchased from Home Depot, Lowes, or similar at 35 dollars for a 4x8 foot sheet, 3/4 inch thick. It is humidity resistant and sturdy, but it can get heavy. They also sell edging in roll form that you apply heat too to make the edges match the rest of the rack if you like a visually appealing rack system. There are loads of DIY videos and guides to help with the process as well.