Substitution for hoisin sauce
Sounds like you don't want to go to a store, so if you're stuck with what you have on hand, probably soy sauce and sugar are as close as you'll get. It won't be thick, but it'll at least have the umami, salty, sweet taste. You could thicken it with corn starch or even flour if the liquid volume is a problem.

Beyond that, the flavor depends on the specific variety, but you might want a little chili and garlic if you have it, and possibly five spice. See also the Wikipedia page on hoisin sauce, which lists some common ingredients.

(Sesame oil doesn't really sound similar at all.)