I have always associated Lady Hel with earth, for obvious reasons. I don't think I've ever seen a sigil for her though. Most of the Nordic people view her with more than a little trepidation. The Germans took a somewhat more favorable view of her, viewing her as symbolic of a gentle transformation through death. Being Nordic myself, while I do not fear Lady Hel and view her as a gentle guardian goddess myself, I do tend toward the Nordic writings. I have yet to find the Nordic writer who is in a hurry to contact Lady Hel right this instant. LOL!
Have you checked demonolatry.org for a sigil for her? They have Connolly's book in .pdf and sigils to accompany the text posted there. There could possibly be a sigil posted for her there. If not, just remember that she is half alive, half dead herself. Her face "half in dark, half in light". Still, I personally do not feel comfortable making things like sigils or bind runes myself and usually leave such things to someone with more experience with such things.
Remember that the overwhelming majority of the "demons" that are named are not demons at all and never have been. They were just deities much older than YHWH later maligned by the Abrahamic faiths in an attempt to get people to quit following them. I've never heard the name of a "demon" that I could not place almost immediately as a deity previously honored or worshiped as a benign deity that was later viewed as "evil". Connolly is absolutely correct in that. That's hardly surprising though as the Abrahamic faiths tend to view anything not of their own faith as "evil". The concept of a "demon" is entirely a tool for social control as far as I'm concerned.