For Roxy to poof in a working slab, she would probably have to immerse herself in the aspect of the bed she was trying to conjure, similar to when had to immerse herself in the idea of the Jade caste to poof th matriorb

Definitely, I do wonder if she could do it given enough time and patience, or if the presence of a Muse of Space was strictly necessary to get the Matriorb in the first place.