Check out ER collet chuck Ken.
There are several that you can use to fit your pen mandrel shaft in too or can use for turning Ll sorts of projects.
Take a look here for the collet types, sizes ect.
ER Collets - ER8, ER11, ER16, ER20, ER25, ER32, ER40, ER50 Collets - MariTool
They can have a fitting for MT or specific fitting for threaded lathes.