I've made the same thing using 1/4" thick glass tile. I lap them flat on SiC loose grit then use very thin very uniform double-sided carpet tape or spray adhesive to stick a thin, fairly stiff "microfiber" cloth to the tile and trim the excess. Afterwards apply your preferred compound and voila. I have about 4 of them now, a CeOx, a .1u CBN, a .25u CBN and a .05u PCD. I don't use them a whole lot, but every now and then.