Styrene sheet (plasticard) is available with various embossed patterns, including simple tile grids. Trace your base rim, cut out the rough shape, glue to the top, then trim/file flush for a seamless edge.
If you don't feel like shopping, there are DIY alternatives that don't take all that much more effort, from scribing your own lines in plain styrene to sculpting tops in
GS (or another epoxy putty, air-dry clay, etc.) to carving the surface out of sheets of pre-baked polymer clay (James Wappel uses this technique to great effect, as seen on
his blog).