The Norse did not use symbols. They were primarily oral people that did not use writing. Writing was often seen in Norse culture as something for "wizards" or other evil beings. The vast majority of Norsemen during the viking period were completely illiterate.
For a modern-day, literate person, the logical symbol for Loki would be mistletoe, a sprig of which Loki used to kill Baldur by the hand of Hodur, his brother.
Also, many modern fictional representations of Loki show him with an elaborate horned helm, so you could use that too.
A couple of posts have suggested that Loki was associated somehow with particular runic letters. This idea is absurd and untrue. Neither is there any evidence for this idea, nor does it make any sense in a cultural context. Loki is mentioned in one of the rune poems, but to infer from this that the letter involved was his "symbol" is not sensible.