\documentclass[portrait,fontscale=0.7,margin=0.7cm,paperwidth=15truecm, paperheight=9truecm,debug]{baposter}


\begin{document}

\begin{poster}{
  background=plain,
  bgColorOne=lightgray!30,
  headerheight=0.4\textheight,
  columns=2,
  headershade=plain,
  headerColorOne=green!40,
  boxColorOne=lightgray!75,
  headershape=smallrounded,
  textborder=roundedsmall,
  linewidth=0.5pt,
  borderColor=green,
  headerborder=open,
}{\fbox{\parbox{1cm}{Logo}}}{\fbox{Poster Title}}{\fbox{Poster authors or subtitle}}{\fbox{\parbox{1cm}{Logo}}}

\begin{posterbox}[column=0]{Box 1}
Text of Box 1
\end{posterbox}

\begin{posterbox}[column=0,below=auto,height=bottom]{Box 2}
Text of Box 2
\end{posterbox}

\begin{posterbox}[column=1,height=bottom]{Box 2}
Text of Box 3
\end{posterbox}

\end{poster}
\end{document}