Hey DharmaBrother,
There is no sigil that I have heard of, Dorje Shugden doesnt only respond when you have his correct sigil, but he responds to all sincere requests for help IF it benefits the requestor. You can use an image of him like a statue or thanka, you can also use a cheap print of him, or even a picture on a website (like this one) .... even if you can visualise how he looks, that is alright also.
Our protector is interested in Mind Transformation, the teachings of the Lamrim, holding our vows, upholding guru devotion.... in your case for example, request for his help to further your spiritual path, and remove obstacles along the way... if your girlfriend is a so-called 'obstacle' to your spiritual practice, Dorje Shugden may help you by slowly changing how she feels about your practices. That is, of course if you keep up your commitments and really do practice. He will not do anything to harm her, He is a BUDDHA and will never do anything of that sort...
He has done the same for me in the past, and I have full faith in his ability to help you too.