I have several "original" piano shawls from the 19-teens or 1920's.
They were gifts from my mother who received them as new items.
Uprights piano shawls are rectangular, with or without fringe.

My grand piano shawls are triangular and designed so that the front
point of the shawl hangs over the front of the piano.  The rear is
folded to align with the open forward third of the lid.

Shawls were also made as a trapezoid with a front triangle and flat
base. I have also seen shawls that are square.

The material is your choice. The shawls I have are brocade, crepe
with double embroidery, and satin.  All of them have fringe.  They
are treasures.

Hope my information is useful,

Ray Bauer
Tahoe City, California -- looking out over lovely Lake Tahoe