Mark Kinsler wrote, "I don't think that there's any good solution."

The obvious solution is proffer a carefully crafted explanation of
the various reasons why it would be, in your expert opinion, extremely
disadvantageous to perform such an alteration on the cabinet.  If
nothing else makes an impression, the fact that such an alteration
would severely affect to value of the instrument might win them over
to your point of view.

If the customer in recalcitrant and unreachable, then it becomes a
matter of how hungry you are.  If you can afford to pass up the job,
then graciously decline on the grounds that you just cannot, in good
conscience, defile the instrument and take your leave.  Don't leave
in a huff; rather, leave the door open in case they have a change of
heart, or would need other work done.

After they look about for another technician and have more time to
think about it, and possibly run into further resistance to their plan,
the customer may still come around.

Tom Lear
San Francisco, CA