All my Squares are painted on reused/repurposed plexi.  In their first life they were used to protect album covers lining the museum wall at Carnegie Hall.  When the Hall renovated they were replaced.  So for the squares it’s a second life and for me a great reclaimed object to paint.  They’re solid, clear, and a consistent shape – square – literally the size of an old LP.  I paint both sides of the plexi which allows for depth and layering.