What is required for the validity of voluntary prayer inside or on the Kaaba regarding facing a structure?

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The established opinion is that requiring a structure to be in front of the worshipper is not stipulated, because the obligation is to face its location and its space, not necessarily its walls. The evidence for this is that if the Kaaba were destroyed, the prayer would still be valid facing its former location. Similarly, if one prays on a high mountain far from its perpendicular alignment, the prayer is valid facing its space, and the same applies here regarding the Kaaba's interior space.