That's a paper board.
Could a blank paper, no lines be used? Little blue triangles on the edges as indicators. We don't know because I was too lazy.
9 x 9 inches. It's an ordinary piece of paper. This happens to be good dimensions.
Measurements in centimetres: 22.7 % 7 = 3.24 mm. 3,24, 6.48, 9.72, 12.96, 16.2, 19.44, 22.68/22.7.
Posterboard is a bad material for a game board. But you know, it might work out for this. Don't expect the corners to last.
So how about this: A proper commercial board, white, with no lines. The pieces go in pits, which would make it easier to centre them after tossing them on the
board. This works in theory, maybe not in practise. The board would be made of .. you would think I would know that by now.
Yellow has moved, blue has no options.