If Heaven meets the requirement of "a somewhere to exist" then wouldn't that mean Heaven is part of the Universe?
I guess that we have to decide whether 'the universe' means
1. The observed spatial-temporal continuum, along with all of its physical contents.
or...
2. Everything that has being or exists in any way.
These two meaning of 'universe' are often treated as if they were identical and co-extensive, but they aren't.
For one thing, it's possible to imagine multiple disjoint space-time continua. In other words, even if we had a TARDIS capable of traveling to every point in space and time that's connected to 'here' by a continuous path, there might be places where we can't go, because no continuous space-time path connects 'here' and 'there'. (If we imagine a space-time continuum as a 4-dimensional bubble, a different continuum would be a second bubble that shares no points in common with this one.
It isn't totally outlandish, many physicists speculate about 'multiverses'.
OK, so is a separate space-time continuum part of this universe or not? That depends on how we define 'universe'.
If we define 'universe' to mean space-time continuum, then the answer is no. If we define 'universe' to mean anything that exists, then the answer is yes.
Applying this to God, if we say that God created the universe and mean 'universe' to mean 'this space-time continuum', then even if we insist that God has to be located somewhere (most theists would probably dispute that), we can easily say that he is in another continuum. (Call it 'heaven'.)
But if we mean 'universe' to mean 'everything that exists', then the idea that God created the universe has deeper problems.