Etharion has the answer and the Pseudo-Silmaril.
Why did the Green-elves of Ossiriand object to the presence of Men in their land
I didn't remember that Telchar also made the Dragon Helm of Dor-L’min, but that makes more sense than Robert Foster who just said it was made by Dwarves. I couldn't find anything in
the Silmarillion about its maker; however I found its full description on page 80-81 of the Ballantine edition of
Unfinished Tales. There it does give Telchar as the Helm's maker and Foster's mistake was probably caused by it having
'a visor (after the manner of those that the Dwarves used in their forges for the shielding of their eyes) and the face of one that wore it struck fear into the hearts of all beholders, but was itself guarded from dart and fire.' It goes on to tell of the dragon crest in the image of Glaurung.....