John McClelland offers a good solution to making a belly cloth.
Having made up a number of belly cloths over the years, I concur with
his approach to making a pattern where the original is missing; it
works equally well regardless if the piano is on its feet or its side.

In addition to John's suggestion for material, I have found that
my friendly neighborhood fabric store stocks a fine quality vinyl
sheeting that is both quilted and padded, making it a superb
substitute.  It's easy to work with, and easy to make a strong double
layer edging to support replacement snaps.  Found in the upholstery
section of the store, and available in a number of colors and styles.
Its mahogany emulation is very appropriate.  Adds a nice touch of
elegance.

Regards,
Terry Smythe
Winnipeg, Manitoba, Canada