These blank panels are made of printed circuit board material and are 1.6mm thick. Perfect for filling in those gaps in your case. One side is patterned while the reverse is pure black (the pure black sides can have slight imperfections because of the manufacturing process). Note, the 2hp blank is not black on the reverse graphic side.
The 2hp and 4hp designs are based on a Turkish carpet motif representing ram horns (boynuz): a symbol of power.
The 3hp and 6hp blanks are designed using the hilbert curve fractal.