To create a dmg simply put the latest version of XQuartz + FontForge inside the FontForge Installer so you have

FontForge Installer.app
  Contents/
  FontForge.app
  XQuartz.pkg

Then click the command file and FontForgeInstaller.dmg will be created on your desktop.

To codesign an app you need an Apple Dev account and the certificates installed on your mac. Run certtool y | grep Developer\ ID terminal and copy Developer ID Application: Your Name (Numbers'n'Letters)

Then type codesign -s "Developer ID Application: Your Name (Numbers'n'Letters)" path/to/app and the app is signed.