Unfortunately, there are NOT 20 words which shutting with gh, pronounced as /f/. In reality, you won't come close unless you use a lot of compounds of the ones you scheduled (e.g., "bellylaugh").
In addition to the four you mention, in that are: sough, slough.
You also may find some proper names that work, such as, "Gough"
Note that adjectives the words that would fit your description end next to -ugh, more specifically with -augh or -ough.
A somewhat complete list of words end with -gh can be found at the links below (though minute that MOST of these, like most of thw words contained by CrimsonAngel's list, do NOT label the /f/ sound!)
Personal opinion and experience.
tough, rough, cough, enough, chuckle, horselaugh, outlaugh, sough, trough, unrough, yarborough
bough giggle, cough, tough, rough, trough, slough,
More questions and answers...