These piano caster cups
are made of lucite, a strong acrylic product that is nearly unbreakable. A newer product, essentially replacing plastic caster cups, we are very impressed with the lucite, and highly recommend these piano caster cups.
Given the strength of the material, the manufacturer is able to make the caster cup relatively thin compared to wood caster cups (obviously a thin piece of wood would crack with the weight of your piano). The smaller bottom of the lucite caster cups makes it absolutely perfect for upright pianos.
The caster cups are also available in a clear lucite: Clear Lucite Piano Caster Cups
Piano Caster Cup Dimensions: