I want to typeset the numero sign № in Plain TeX.
My first guess is to use
\def\numero{$\rm N^{\underline o}$}
but the line under the o is too long and I don't really want to mess up with arbitrary length values to shorten the box.
And
\def\numero{$\rm N^{\b o}$}
does produce a normal-size o which is not what I want.
Any idea? I am fine with the o being superscript or baseline.