I've only been able to find regular umbrellas. Is there an inexpensive source for square ones anywhere locally?
Anyway, if it is studio umbrellas, then maybe can sewblack cloth along the edges of a regular round (or octagonal to be exact) umbrella and create a square mask. Alternatively just get a square soft box.
As complexity rises, precise statements lose meaning and meaningful statements lose precision.
one place currently in mind is CP.