some time ago i addressed the "fall off" issue on a strip light soft box
by using a deflector together with the soft box.
try both a semi transparent and intransparent one - took mine (
example)
from my EL beauty dish, plugged it in, mounted the box, made a huge difference (cant remember exact measures center / edges though).
alternatively any suitable disc could be DIYed with pieces of wire to the speed ring -
for a totally customized result.
cheers