Looking to do some work with the Greek Gods like Dionysus and Athena. Any pointers or recommendations?
Gods do not usually have sigils. The traditional method of contacting them is through prayer and oration and devotion.
Call them through their mythology. Make offerings. Meditate on them and seek their presence. The gods are not contacted the same way as demons or angels. Sigils are not required.
Also, sigils are a relatively new invention in spirit communication. The use of an idol or prayers like @DarkestKnight suggested has been used for much longer.
But if you want to use a sigil, there are sigil generators where you can type in the name and poof! Sigil! There was one shared on here recently.
Or you can draw their names on a planetary kamea, all of which are available via search engines.