\begin{tcbposter}[
  coverage = {
      spread,
  },
  poster   = {showframe=true,
    columns=4,
    rows=2,
  },
  boxes    = {
    left=2pt,
    colback=white,
   },
]

\posterbox[blankest,rotate=180, fit, fit basedim=10pt]{name=first,column=1,row=1,span=1,rowspan=1}{
  {
    \rotatebox{90}{
      Back
    }
  }
}


\end{tcbposter}