This lesson is short and sweet...
I've made what is essentially a 'filter holder', adapted to the enlarger.

This is an 'exploded' view.

All ya need is: black matteboard, an exacto, and some black tape.

The top 'tray'/"filter holder" is about 1.5 inches below the lense.
( Good place for a contrast filter )

The lower one, about 3 inches below the lense.

Insert a piece of black construction paper that's larger than the opening for light....

The degree to which the image is vignetted is determined by f-stop - and, to a lesser extent, which tray you use.

You can cut, and arrange, the black paper any way you want....and, wow, does it ever get alotta 'basic dodging' done...

Obviously, further improvisation is up to you.....:-)

....