The point of a diffusion scrim is to spread the light, so you take the
original 2 inch or so light source, the flash tube, and make it much larger.
The point of putting a reflective backing is to redirect the flash towards
the subject.   That was to fix a problem they used to have about having
enough flash power.

What is the point of modifying light?  Is it make boxes or to put light on
subjects?  BTW, back when photogs used parabolic reflectors to focus
tungsten lights they were used about 36 inches from the subject.  When
umbrellas were introduced with the first flashes, they were about twice as
large but used about six feet away.  soft boxes allowed the photog to bring
the light closer so a 3 foot softbox used at 3 feet was twice as large as
the brollie.  but photogs are shooting much fewer head and shoulder close
ups and far more half and full lengths.

If we want to light subjects we should decide what kind of light works best.
General rule of thumb that commercial photogs use is to make the light twice
as large as the subject, of course while they are lighting small to large
objects.   Remember, what's the nicest of natural light, large picture
windows, porches, overhangs, open shade, twilight, all super large light
sources, light that comes from a large arc.

Are we limited by low power flashes?  probably not.  You have a shower
curtain, hang from the ceiling with thumb tacks, aim the light through it,
now you have a large light source.

you can use styrofoam coolers, you can use sheets of styrofoam insulation
panels, put three in a wide triangle, hang your curtain from the front with
the flash inside.

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