Easiest way to make a soft focus filter is to take a skylight or UV filter
(I buy ones with dented front rings for this-never paid over $2 for one)
and
spray they with hairspray. If you don't get the effect you want-ie too
soft
or not soft enough,just soak it in rubbing alcohol for about 2 minutes and
the hairspray washes off and you can try again.

as one who went through the whole "buy top quality tack sharp lenses for my
medium format gear only to do many things to soften it" routine, hair spray
is one of those things that mushes up the focus which is not the same thing
as softening it.

GOOD soft focus retains a sharp image. Good soft focus looks better as you
enlarge it. Mushy focus looks ok rather small but falls apart when
enlarged.

The best cheap soft focus is two layers of cheap nylon netting, (not
expensive hosiery, the good stuff adds to much warmth to the image.) Black
netting will absorb some light rays on a seemingly random pattern (if tight
enough and that's why you want to use two layers at least) and diffract
light passing close to the edges of each thread, that's what makes the image
partly soft. White netting works similarly but seems to do more with the
highlights and lowers contrast more.

one trick I heard about but never tried was to take a cheap UV filter and
apply tiny dots of clear glue in a random pattern.

I eventually used Harrison Black dots (Which actually looked like someone
spit chewing tobacco between two sheets of glass) and then invested in a
real soft focus lens which I consider one of my better investments in image
gadgetry.

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