I learned this trick over in the model rocketry newsgroup, maybe
it will help.  Over there, they make there own decals using ordinary
gummed packing tape or similar water-soluble material.

Place the tape gummed-side-up, and spray it with several coats of
varnish or polyurethane, then paint, draw, trace, or print the desired
decal on it, then add several more layers of varnish or poly.  Cut out
the decal, place in water, and soon it will loosen.  Place it where you
want it, slide it into position, pat it dry with a soft cloth (prick
any air bubbles that form using a pin) and when it has dried, varnish
or poly over it again a few times.

This method has had great success with model rockets, anyway.

Regards,

Larry Smith