Hallo Freeglers Far and Wide!
Hoping to strike gold with this request, before these 3 pieces, held in place by the stone, strike um domestic gold below! (It is a Victorian cess pit, if that's not too much information).
Measurements (from that era) are Imperial: 20x26inches.
Something more recent at 510 x 660mm might do it...
If you are upgrading an older property, thanks for checking your scrap pile!