Your octopus will decide which areas it likes for a den but it will pick somewhere that you cannot see in. Typically, we try to aquascape for our own esthetics and often the octo will reject the area we have made, ie there is no known ideal setting, but will find or make a hiding place somewhere in the rockwork. You can provide a few loose ping pong ball and smaller sized rocks and shells that the octopus can easily move around to fashion doors,
Looking through the journals should help with ideas but here is a link to Maya's journal and a video of her moving a gorgonian to the front of her den. Just before doing this she had changed dens to an opening in the live rock across the tank. She decided she liked her original den better but wanted the "plant" that was in front of the abandoned new home. Sadly, all this effort was to establish a den to lay eggs but it was fascinating to watch.
Looking through the journals should help with ideas but here is a link to Maya's journal and a video of her moving a gorgonian to the front of her den. Just before doing this she had changed dens to an opening in the live rock across the tank. She decided she liked her original den better but wanted the "plant" that was in front of the abandoned new home. Sadly, all this effort was to establish a den to lay eggs but it was fascinating to watch.