It is adviced to use the prescribed sigils for the gods, and if there are no sigils given. It is usually so that the god(dess) in question doesn't wish to be contacted.
But I myself am rather experienced in sigillic magic, the magic that uses self designed sigils to realise goals.
If it is the case that you really need a specific god of which you can't find the sigil, and there is no other god that will do. You might be ethically allowed to call upon him/her with a sigil you devised yourself. Also, gods that don't have sigils because they might be not popularly known, or gods that just aren't listed on JoS (Example: Thanatos) you can create your own sigil. If it works, is still questionable though.
The gods probably won't get mad at you, but stick to protocol as much as possible on this one. Why take the hard road when the easy one is just as effective, efficient and much safer?
Ave Luciferi
