I build my own, so I have extensive experience. I form a block of pine to the exact dimension and contour, coat it with black shoe dye (important - lacquer-based dye), insert the block in the tender coal bunker, and then carefully apply a heavy thick coat of Weldwood contact cement (like Walthers Goo, but cheaper and better). Then I quickly spread coal or fake coal in a thick layer over the glue, and go away for 24 hours.
Next day I turn the tender upside down over the same newspaper that was under it when I spread the coal, and return most of it to its container.
During painting operations, if I am spraying black or Dullcote, I let the paint hit the coal. Photo if you want.
