Roger,  I would use the thinnest split skiver I could get.  Even this
will probably be thicker than the original.  You may have got your
thin tosh too tight.  Try releasing it a bit and the pneumatic may
lie parallel.

You may have to use zephyr.  I use this on organette pneumatics of
small size which are probably about the same dimensionally, as the
Celestina was made by the same firm as the Grand and at about the
same time in history.

Best regards,  Nicholas Simons