You might try:
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https://www.amazon.com/gp/product/B07MM185Q2?pf_rd_r=QF4YTXJHX4P4MSFQRH68&pf_rd_p=edaba0ee-c2fe-4124-9f5d-b31d6b1bfbee
They're 100mm x 100mm x 3mm birch plywood squares. Only takes minimal work to get it to work as a lens board.