Rectangle with points uniformly distributed inside in Tikz

I'm trying to draw a rectangle wit random points inside using tikz. My attempt is this:

\documentclass[tikz]{standalone}

\usepackage[T1]{fontenc}
\usepackage{lmodern}

\usetikzlibrary{calc}

\pgfmathsetseed{20}

\tikzset{
    particle/.style = {draw,circle,inner sep=0,outer sep=0,minimum size=3}
}

\tikzset{
    pics/PE/.style
        2 args={
            code={
                \node[
                draw,rectangle,
                minimum width=80,minimum height=40] (main) at (0,0) {};
                \foreach \i in {1,...,#2}
                {
                    \path let \p1 = (main.south west), \p2 = (main.north east) in
                    node[particle] at 
                    ($ (main.south west) + ({random(\x1,\x2)},{random(\y1,\y2)}) $) {};
                }
            }
        }
}

\begin{document}


    \begin{tikzpicture}
        \draw pic {PE={1}{30}};
    \end{tikzpicture}


\end{document}

...but this doesn't work. It seems I cannot use a coordinate inside a call to the random function. Is that so? Any workaround?

Cheers!!