Change margins for tcolorbox with varwidth environment

In the code posted below, I am using varwidth to shrink my tcolorbox to the text width for short text lines. However, I also have some wraparound text lines and the tcolorbox is too big in that case. How can I retain the shrink-to-fit behavior on short text snippets while changing the maximum margins of the tcolorbox? I would like the maximum margins of the gray area to be slightly less than the regular paragraph margins on both sides, but I also want to keep is shrinking to fit when the text length is smaller than that.

\documentclass[preprint]{revtex4-2} \usepackage{lipsum} \usepackage{setspace}  \usepackage{varwidth} \usepackage{tcolorbox}\tcbuselibrary{breakable}\newsavebox{\myboxbox}\NewDocumentEnvironment{mybox}{+b}% find a better name{%    \centering    \setlength{\parindent}{0pt}%    % measure the text width    \sbox{\myboxbox}{\begin{varwidth}{\columnwidth}#1\end{varwidth}}%    \begin{tcolorbox}[        breakable,        width=\dimexpr\wd\myboxbox+8mm,        top=3mm,        bottom=4mm,        ]        #1    \end{tcolorbox}%}{}\begin{document}\lipsum[1]\\\begin{mybox}    \begin{singlespace}    \lipsum[1]    \end{singlespace}\end{mybox}\begin{mybox}    \begin{singlespace}        Just a few words here.  Just a few words here.    \end{singlespace}\end{mybox}\end{document}