WW2 Guadalcanal Japanese Officer Headquaters?

Hmmmm... good question. I suppose it all depends upon which phase of the battle you prefer to depict. The early portions were something of a meeting engagement with no prepared Japanese positions. I imagine that their engineers and enlisted HQ troops created some sort of jungle hooches with overhead concealment and rain protection for the headquarters during this phase. Later when they were on the defensive, they did have prepared positions with dugouts and such. Most of the island was wilderness aside from some coastal plantations and a few native villages, so western structures would be few and far between. Native huts could have been used for HQ purposes.

Waist deep in water. The principal General Officers were Hyakutake and Imamura. There are plenty of images online of them, generally standing outdoors in very formal poses.

I also would guess that the more permanent installations were still at Rabaul.

https://www.history.navy.mil/research/library/online-reading-room/title-list-alphabetically/g/guadalcanal-campaign.html

has a lot of information about the Japanese locations of forces.

There's a recon map in that document probably drawn from interviews with natives showing the area around Lunga Point. Numerous hand notations refer to "houses".

Native houses there looked like this. Thatched roofs, bamboo walls, pretty respectable structures.

Any of those should work.

Officers, no matter from what country, as a general rule will appropriate the best of what's available.  There are exceptions though.

There is, of course, the "pagoda" which was constructed by the Japanese prior to invasion at Henderson Field.  It was HQ for the troops there at the time.

https://en.wikipedia.org/wiki/File:Henderson_Field_Pagoda_flight_operations_headquarters_on_Guadalcanal,_October_1942_(USMC_50921).jpg

Gary