Create a box around a hidden text in a Beamer presentation

Question

I used the solution mentioned here https://tex.stackexchange.com/a/225079/75983 to hide parts of my presentations in the handout version of the slides. I am wondering if it is possible to place a box around the empty space with a special sign so that students can figure out there is a hidden text that need to be filled. The space should be on the same size of the hidden text and take care if the text is in a single or multiple line. Here is a MWE:

\documentclass[16pt,notheorems,handout]{beamer}
\newcommand\handoutgap[1]{\invisible<0| handout:1->{#1}}
\usepackage{beamerthemeshadow}
%%%%
\title[MWE]{Test}
\author[Author]{Author}
\date{\today}


%------------------------------------------------------------%

\begin{document}

\begin{frame}
\frametitle{Test}
Single line equation
\handoutgap{
\[ x^2 + y^2 = z^2, \]
}

\end{frame}

\begin{frame}
\frametitle{Test}
System of equations
\handoutgap{
\begin{align*}
  x^2 + y^2 & = z^2 \\
  r^2 + t^2 & = m^2 \\
  s^2 + k^2 & = n^2
\end{align*}
}

\end{frame}

\end{document}

Answer 1

You could use tcolorbox for it. For the special sign you may want to have a look at the overlay option, but usually a frame should suffice to say "Fill it". Besides I consider it a good idea to give some more space (e.g. the line width) for students to fill something in. But you can set the width of the box using the width key.

\documentclass[16pt,notheorems,handout]{beamer}
\usepackage{tcolorbox}
\newcommand\handoutgap[1]{\tcolorbox[arc=0pt,colback=white,size=fbox,colframe=black]\invisible<0| handout:1->{#1}\endtcolorbox}
\usepackage{beamerthemeshadow}
%%%%
\title[MWE]{Test}
\author[Author]{Author}
\date{\today}


%------------------------------------------------------------%

\begin{document}

\begin{frame}
\frametitle{Test}
Single line equation
\handoutgap{
\[ x^2 + y^2 = z^2, \]
}

\end{frame}

\begin{frame}
\frametitle{Test}
System of equations
\handoutgap{
\begin{align*}
  x^2 + y^2 & = z^2 \\
  r^2 + t^2 & = m^2 \\
  s^2 + k^2 & = n^2
\end{align*}
}

\end{frame}

\end{document}