Shimano freehub into fixed

There's an easy way that works for freehubs and multiple freewheels.
I did it several times, always worked well and also easy to convert it back.
Just open the locknut of the bearing system till the pawls are visible.
Push some small pieces of nails, spokes, e.g. behind the pawls to fixate them.
Put the locknut back in place, assemble the wheel, set a sprocket & spacers in such a way that chainline is okay. Job done.

You dropped your screwdriver...