I borrowed the following MWE from here but I would like to be able to create an empty environment to use for exercise answer with variable vspace. The exam class has the \makeemptybox{} option, but I wish to use the tufte-handout class and the mdframed package because it allows page breaks.
\documentclass{tufte-handout}
\usepackage{tikz}
\usepackage[framemethod=default]{mdframed}
\global\mdfdefinestyle{exampledefault}{%
linecolor=lightgray,linewidth=1pt,%
leftmargin=1cm,rightmargin=1cm,
}
\newenvironment{mymdframed}[1]{%
\mdfsetup{%
frametitle={\tikz\node[fill=white,rectangle,inner sep=0pt,outer sep=0pt] {#1};},
frametitleaboveskip=-0.5\ht\strutbox,
frametitlealignment=\raggedright
}%
\begin{mdframed}[style=exampledefault]
}{\end{mdframed}}
\usepackage{blindtext}
\begin{document}
\section{Some text}
\blindtext\footnote{text ends here.}
\bigskip
\begin{fullwidth}
\begin{mymdframed}{This empty frame} % requires a vertical space of 25cm
\end{mymdframed}
\end{fullwidth}
\section{Another text\protect\footnote{to be continued after the empty frame with about 25cm}}
\blindtext
\blindtext
\end{document}