IKEA sells nice and cheap glass coasters
for use as plates for block candles. These have a nice and flat backside, perfect for etching in circular designs. In particular, I have had a LOT of fun etching in hyperbolic tilings
One word of caution: I tried measuring the coasters, ending up with a diameter of 110mm. IKEA states 4.25". Neither is actually correct — best results requires using a diameter of 4.288" in the design. As a result a few of my coasters have overlaps, where the last few mm of a design are etched onto the next coaster in the laser cutter.