Is that the "Micro Billet Third Brake Light (EL-MB)"?
I ordered the "Retro Round Micro 3rd Brake Light (EL-MBRX)" this weekend. Was originally going to have a third brake light molded into the top and sealed with the clearcoat. The rodshop owner talked me out of it, said he cannot guarantee it would not leak and onec done there would be no way to access it. He suggested one that bolted in the rear window. Spent many hours looking at different ones and it was a toss up between Watson's "Small Billet Third brake light" and the "Retro Round Micro 3rd Brake Light (EL-MBRX)". Price of the billet version seamed excessive. After seeing your post I think it was the right solution.
If you have the EL-MB version, please provide the actual dimensions, they gave the dimensions of the EL-MBRX version but only showed the EL-MB beside a quarter.

Neil