Zoos in Australia with Platypus, Melbourne Zoo, Healesville Sanctuary, Taronga Zoo Sydney, Taronga Western Plains Zoo (with a new platypus facility being built currently, possibly at the end of 2022), Australian Reptile Park and Wildlife Sydney Zoo, those are the ones I know of. Also you can see platypus fairly frequent if you go to Platypus hotspots. Many enclosures in the places mentioned above have a small pond with logs, rocks and aquatic plants, and with some a long stretch water behind a window, but in terms of land space, yeah they really don't need much, tbh the most I've seen is 4x5 in a connected outdoor area to the darkened inside viewing area, but they mostly just have a burrow. Also the depth requirement is monstrous for platypus. Could we just have yabby/crayfish dispenser in the platypus habitats or a worm dispenser that doesn't need such a deep depth, like a horizontal dispenser you put on the side of the body of water, that the yabbies crawl out of, or keepers coming back with more yabbies and worms, to make it more true to the real animal, having its eyes closed underwater too. Platypus don't eat FISH!