A few years ago, I found what I consider to be the ideal belly
cloth material.  I am not sure how readily available it might be
because I bought it at a fabric closeout shop.

The fabric is similar to wetsuit material.  It is knit fabric on
both sides of a neoprene core but is much thinner than.  The fabric
that I found was wool on both sides, one being a low luster black.

There are four major advantages to this material.  First, it blocks
sound very effectively.  Second, it stretches easily to cover lumps and
bumps and makes fitting much less critical.  Third, it is much easier
to sew than Naugahyde, which is difficult to sew without a commercial
machine.  And fourth, it does not need snaps.

The fabric sticks to Velcro (loop side) very well so you can glue,
staple or tack the Velcro hook to the piano and easily remove the belly
cloth without the risk of snaps falling off.  The stretchy fabric makes
it easy to make a cover for the front underside of the keybed which is
very difficult to do with Naugahyde as it needs darts and gussets to
fit neatly.  The stretchy fabric stretches to fit over everything.

To make a belly cloth, you can put the Velcro in appropriate places and
stretch the cloth to fit, sticking it to the Velcro and then cut it
while it is in place.  Even if you get it a little wrong it is fine
since it stretches.

Best regards,
Spencer Chase
Garberville, CA 95542 Postal service only
Laytonville, CA 95454 UPS only
http://www.spencerserolls.com/