I have the one in the firey path, behind the strength boulders. There are three sections to it, but I can only access one. I hope the users of this location have the same shape as mine, because I don't want do wast my money on a decoration that I think can help me get there, but is just a waste of money. How do I access the other areas of my base?
You have to buy Solid Boards, you can get them at the Lilycove Department Store's top floor clearance sale on the roof. I believe they are 500 P$ each, and two should do.
Source: I have the same base you do, and Bulbapedia