The CAMS manual (available online) has all the regs. It is not vehicle specific though. Good cage design has little to do with the CAMS regulations though, the manual only covers sizes and angles and minimum requirements etc.

You can purchase from CAMS a document called a 'homologation paper' This is vehicle specific and documents and specifies all the series production parts that equipped the car. It also shows specifications and pictures of special 'optional' parts and the location of the manufacturer's team built rally cars rollcage attachments. Copying that rollcage design is a good place to start. Chatting to people in the know and looking at well built rally cars is also a good place to gather ideas.

Painting a caged shell is a bit of a nightmare, getting good coverage on all surfaces is a challenge. Don't attempt a clear over base or metallic paint would be my advice. The doors all glass and fuel tank are usually removed for the fabrication and painting process. Ideally the car will be a bare shell on a rotisserie. Some teams go to the extreme of taking cars further down than a bare shell, removing roof, sills and firewall to strip out excess weight before putting back the bare minimum.