Aren't you, by using the word "shape" implicitly assigning a finite size to the Universe? For surely if it has a finite size it would seem logical to assume it has some kind of finite "shape". But if the Universe is not finite, at least not in our ordinary sense of the meaning of finite, then perhaps "shape" is an inapplicable metric. And if "shape" is inapplicable, that is, what it might look like if we were to go outside of it and look at it, then perhaps its true appearance cannot be described by our currently accepted methods of geometry and thus a possible answer to your question is "if the Universe is infinite, it may have a non-geometrical form and thus no shape."