You could see them in the Zoo's Rhinowebcam but not sure if it still works. They are fairly regulation concrete(?) walled large stalls roughly square in shape.
Looking at the photos on Zoochat that depict this exhibit from opening day until now, and seeing how more and more shade has been added, it is fascinating to consider how much more has been learned about Sumatran rhino husbandry over these 20 years.
1989: http://www.zoochat.com/556/sumatran-rhino-1989-a-188147/
Those shade-structures are fascinating! Not pretty maybe, but functional and provide a substitute canopy for a jungle species. Would be nice to see something similar for other species which feel more secure with a canopy overhead (am thinking the Gorilla and Malay Tapir enclosures at London for a start...)