The HPLHS has been at work on an Arkham seal as well, and I've just updated our site with that addition and an update to the Arkham Library Card PDF, as previously promised. Although our seal looks different (and borrows from different sources), it contains many of the same concepts and the two are quite compatible. Both feature agriculture, shipping, and a nod to Miskatonic.
Mr. Leman's interpretation of the seal, and the downloadable library card featuring it, is available over here at the HPLHS website.
Raven, another talented Mythos fan and frequent contributor here, has another take on the seal over here. There's an in-depth discussion of the symbolism and details of it in this discussion thread on Yog-Sothoth.
I love comparing how three different people drawing on the same resources can create similar, but unique interpretations of the same item. I've said before that Mr. Leman was one of the primary inspirations for this site, and his excellent work speaks for itself. Raven has graciously contributed a huge amount of printable paper items to the site, and his in-depth analysis of Mythos minutia has been invaluable.
Embarrassingly enough, I'm still mucking about with my own interpretation of the seal.
Update: Raven put together this very handy side by side comparison of the three seals.