few small issues and questions

I can't seem to access the mailbox to get to the notes players put in.  Do you happen to have a suggested Zone Manager flag layout that works well for this?  That might be a good image to put in the description, as well.   

I noticed that there are a few wallpapers missing in the top room when I was trying to get this figured out, and is there any way to add loot?  My players are a little frustrated that there is nothing at the end, honestly.

Did you get building authorization? That should give you full access, including the mailbox.
(Sorry, I'm not familiar with Zone Manager — is it some kind of plugin?)

The building was thoroughly checked — I'm sure all the wallpapers were intact initially.
(Maybe some went missing after a server restart?)

I usually leave the boxes in the throne room empty so that admins can manually add the most suitable loot.

Changed Status from Pending to Not a Bug

Zone Manager has options to block players from doing actions within a bubble like destroying walls - I use it in my town to keep people from opening doors or destroying walls or repainting signs, etc, I'm not sure how i would get building auth without a TC somewhere, and I didn't see one.  The loot boxes at the top would need to be manually reloaded each time someone completes it, which really isn't ideal.  \

This is a good, challenging design, though - don't let my questions and comments make you think i mean bad.  I'm just trying to see if I can make this work on my server is all.

You can get auto-authorisation when pasting the building -- just add "auth true" to the /paste command. The cupboard for the labyrinth  hidden in the walled column, just a little bit straight from the entrance.

Regarding loot as a reward for players, unfortunately, I could not come up with an adequate way for a player to receive a reward only once. I do not know how to solve this with the standard tools of the game, apparently, a special plugin should be used for this.

And special thanks for mentioning Zone Manager! I'll definitely look into this plugin, it looks like it's a convenient tool to use any PVE events even in the vanilla Rust. Also perfect for more complex events like my Puzzle Houses.