Comment 3 for bug 910454

Yes, your install file should be of the form:
path/to/file/in/package opt/extras.ubuntu.com/<package>/path/to/file

You shouldn't need to use sudo for that, running "debuild" as a user should work just fine.
If you need root access to build your package, it's most likely because it's trying to install in /opt rather than opt/ (where opt/ basically means debian/<package>/opt/) which is a problem as it may do damages on your build system and will most likely not end up in the package.