Diagram of Headquarters
If you've read the text descriptions and seen the illustrations of Headquarters, you have no doubt come to the conclusion that there is no consistent overall picture of Headquarters that can be inferred. Of course, there are certain elements that are undisputed, such as Tunnel Two, the desk, the existence of the laboratory, etc.; but trying to physically place all the landmarks inside Headquarters is challenging indeed. Basically, the office of Headquarters is all that is ever seen with the exception of a single illustration of the lab in "Talking Skull."
Below is my rendition of a likely interior of Headquarters and two 3D renderings of what HQ might look like.
Tunnel Two is seen coming up through the
floor, while Emergency One is seen in the roof above the
desk. Easy Three is most likely behind the green panel on
the opposite wall. I base that on the following evidence:
Door Four is most likely on the wall opposite Easy Three (see the diagram of the Jones Salvage Yard to see why this is probably the case); therefore, although it can't be shown on the diagram above, Door Four is on the first wall through which you look to see the rest of Headquarters. On the few illustrations that show this particular wall, there is also something that appears to be a drape/curtain of some sort that may cover the Door Four panel.
The desk and swivel chair are in one end of the trailer, which is consistent with all the illustrations. I have placed the file cabinet on the same wall as shown in all the illustrations, although you'll notice in the illustrations that it seems to move around quite a bit along that wall.
I have also assumed that the booth illustrated in the stories with the "Laboratory" sign is actually the entrance to the lab and not the lab itself (see my notes on the Headquarters Illustrations page). I know this runs contrary to the illustration in "Crooked Cat," and the text description in "Talking Skull," but it is consistent with the illustrations in "Talking Skull" and seems to make the most sense.
As I stated earlier, it's impossible to be too dogmatic about the exact layout of Headquarters because it now appears that Robert Arthur and Harry Kane had a slightly different image in their minds. But...it's all in fun, and we need to look at it as such.