Try black tinted autobody filler-- it comes either with red or black
hardener - it's been a long time, but a student of a friend made a
checkerboard of cedar squares set into a frame. He just troweled the
stuff in, let harden & sanded it off. If the only stuff you can find
is the red tinted hardener, mix in some black polyester coloring
powder. I can't remember where I got mine, but anyone who sells
polyester casting resin (the clear kind) will most likely sell the
tinting colors that are campatable.
Phil