You are not limited in any form whatsoever. You just need some space in which to work. It doesn't have to be present time physical universe space, it can be past physical universe time space. You just need some space there to work. Now, it doesn't matter where you put the postulate.
Most people find it easier to put the postulate into a mass.
Even a created mass of your own choice or into a wall, a part of the physical universe, a fence, a passing car. It doesn't matter where you put it. That is important. You must be certain that it's in the other's, the class of other's to which I will refer for more precision as the class of not-self. It's you being forced to know.