Why don't you just get like a hooded cape and a pentacle, and maybe a besom and if your trick or treating a cauldron and add your own style to it some magickal alphabets,pagan symbols,God and Goddess symbols,etc...
Thats old fashion stereotype I know but hey that used to be the style.
Like when christianity pushed out paganism and only the peasants who were considered pagan wore the conical (witch hat) because to them it was still in fashion and they couldn't afford the new style which was a subliminal way of christianity saying that paganism is basically the old fasion no one wears anymore. King James is a good example the kings who converted to christianity and tryed to get the rest of their kingdoms and others to join in with the new movement.
But yeah in wicca its very common to wear just that: Hooded cape,pentacle(Or some other pagan jewlery),nice hand decorated hat(optional). Just get into the feeling of Samhain its a time to celebrate and be pagan proud whatever you wear as a magickal costume have fun with it!