\documentclass[tikz]{standalone}

\usepackage{tensorstyle}

\begin{document}

\def\colorPsi{bg-red}
\pagecolor{bg0}

\begin{tikzpicture}[x=\unitCm cm,y=\unitCm cm, scale=\scale, transform shape]
    % for positioning
    % \drawGrid{-1}{10}{-7}{1}
    \pgfmathsetmacro{\N}{8}
    \pgfmathsetmacro{\iMax}{\N-1}
    % Psi with Ms
    \begin{scope}[shift={(0,-1.5)}]
        \foreach \i in {0,...,\iMax} {
            \node[ten] (M\i) at (\i,0) {$M^{[\i]}$};
            \pgfmathsetmacro{\j}{int(\i-1)}
            \pgfmathparse{\j >= 0}
            \ifnum\pgfmathresult=1 \relax
                \draw[leg] (M\i) -- (M\j);
            \fi
            \draw[leg] (M\i) -- ++(0,-0.5);
        }
        % \begin{scope}[on background layer]
        %     \node[fill=\colorPsi, fit=(M0)(M\iMax), inner sep=\dx cm, tensorcorners] {};
        % \end{scope}
    \end{scope}

    % Psi
    \getNodeDimensions{M0}
    \begin{scope}[shift={(0,0)}]
        % \draw[ten] ($(0,0)-(M0.width)+(M0.height)$) rectangle ($(0,0)+(M9.width)$);
      \draw[ten] (-0.5*\nodeWidth,-0.5*\nodeHeight) rectangle ($(\iMax,0)+(0.5*\nodeWidth,0.5*\nodeHeight)$) node[midway] (psi) {$\psi$};
      \draw[leg] (psi) -- ++(0,-0.5);
    % \begin{scope}[on background layer]
    %     \node[fill=\colorPsi, fit=(M0)(M\iMax), inner sep=\dx cm, tensorcorners] {};
    % \end{scope}
    \end{scope}

    \node at (\iMax/2,-0.9) {$\approx$};
\end{tikzpicture}

\end{document}
