How can I write blackboard bold Latin capital letters in Plain TeX?

In LaTeX, you can use commands like \mathbb{C} (in amsfonts) or \mathds{C} (in mathds) to get blackboard bold letters. But how would I get them in plain TeX? I tried {\ds C} and it didn't work.

Just standard setup; the fonts are msbm10 and related ones.

\font\tenamsb=msbm10 \font\sevenamsb=msbm7 \font\fiveamsb=msbm5
\newfam\bbfam
\textfont\bbfam=\tenamsb
\scriptfont\bbfam=\sevenamsb
\scriptscriptfont\bbfam=\fiveamsb

\def\bbb{\fam\bbfam}

${\bbb N}$ is the set of natural numbers.

\bye


Of course, for serious math typesetting you should use AMS-TeX

\input amstex

$\Bbb{N}$ is the set of natural numbers.

\bye


\font\mbb=msbm10
\newfam\bbb
\textfont\bbb=\mbb

$x \in {\fam\bbb Z}$

\bye