Michael ReesYou just eyeball it to make a cardboard template for the ends and measure the width. Don't try to make pockets for the platen knobs or the return lever. Just make the whole thing loose enough to cover the protrusions. I use 20 gauge sail window plastic so the cover is transparent.
Via Michael Rees at https://www.facebook.com/groups/TypewriterCollectors/posts/10163310003194678/
and specifically recommends https://www.sailrite.com/Plastipane-20-Gauge-Vinyl-Window-Material-54