That is not going to work as you are still creating a self signing certificate.
You need to get a Developer Certificate from symbian signed. It will not work with a self created certificate.
Then just sign the supplied sis file with
signsis -o -s -v gnubox.sis gnuboxs.sisx developercert.cer developerkey.key password
You need to get a Developer Certificate from symbian signed. It will not work with a self created certificate.
Then just sign the supplied sis file with
signsis -o -s -v gnubox.sis gnuboxs.sisx developercert.cer developerkey.key password