You could, of course, use TikZ for this:
The symbol will scale with your font size, since it uses ex
to define the path.
\documentclass{article}
\usepackage{tikz}
\begin{document}
\newcommand\shield{%
\tikz [baseline] \draw (0,1.75ex) -- (0,0.75ex) arc [radius=0.75ex, start angle=-180, end angle=0] -- (1.5ex,1.75ex) -- cycle;%
}
A shield: \shield
\end{document}
If you're feeling fancy, you could parametrise it a bit:
\documentclass[border=3mm]{standalone}
\usepackage{tikz}
\begin{document}
\newcommand\shield[1][]{%
\tikzset{
shield width/.store in=\shieldwidth,
shield width=1.5ex,
shield height/.store in=\shieldheight,
shield height=1.75ex
}%
\tikz [baseline,#1] \draw (0,\shieldheight) -- (0,\shieldwidth/2) arc [radius=\shieldwidth/2, start angle=-180, end angle=0] -- (\shieldwidth,\shieldheight) -- cycle;%
}
A shield: \shield
A wide shield: \shield[shield width=2ex]
A tall shield: \shield[shield height=3ex]
\end{document}
The Unicode symbol for ₣ is:
U+20A3 FRENCH FRANC SIGN
Libertine is an example for a Unicode font that supports the character:
% lualatex or xelatex
\documentclass{article}
\usepackage{libertineotf}
\begin{document}
^^^^20a3 % ASCII notation
\end{document}
Example for GNU FreeFont:
\documentclass{article}
\usepackage{fontspec}
\begin{document}
\fontspec{FreeSerif.otf}
^^^^20a3
\end{document}
Another example for Times New Roman (from Windows 7):
\documentclass{article}
\usepackage{fontspec}
\begin{document}
\fontspec{times.ttf}
^^^^20a3
\end{document}
\fontspec{Comfortaa-Regular.ttf}% Comfortaa Regular
:
\fontspec{OpenSans-Regular.ttf}% Open Sans Regular
:
Example for siunitx
and pdfLaTeX
If lualatex or pdflatex cannot be used, then the symbol can be included as graphics. The graphics is generated by:
% franc.tex
\nofiles
\documentclass{article}
\usepackage{fontspec}
\begin{document}
\fontspec{FreeSerif.otf}
^^^^20a3
\end{document}
It is compiled via:
$ lualatex franc
$ pdfcrop franc
Then franc-crop.pdf
can be used as image for pdfLaTeX:
\documentclass{article}
\usepackage{graphicx}
\usepackage{siunitx}
\DeclareRobustCommand*{\myfranc}{\includegraphics[scale=.1]{franc}}
\DeclareSIUnit{\franc}{\myfranc}
\begin{document}
\SI{123.45}{\franc}
\end{document}
If you are using lualatex or xelatex, then the font can be used directly, e.g.:
\DeclareRobustCommand*{\myfranc}{%
\begingroup
\fontspec{FreeSerif.otf}%
^^^^20a3%
\endgroup
}
Best Answer
And a TikZ version: