20 W x 13-3/4 D x 1-1/2 H made from the same 3/4 inch MDF as the rest of the cabinet. Painted black.
EDIT: correction, only the edges of the riser are painted. The outer surface is a textured laminate of some sort, probably melamine. It's the same material/finish used on the risers and collars between the LF and HF sections of models 14 and 19.
They also have feet/glides on the corners of the riser from the factory. Nearly white in color, i think they're nylon, 3/4 diameter, about 1/4 inch tall