You can often get these for a reasonable price from local hospitals or from medical suppliers of refurbished equipment. Depending on how leaky they are and how much oxygen you pump in you can get high inspired oxygen concentrations. Now that I think of it, an oxygen sensor is a good bit of kit to have if you use oxygen cages with any frequency. They are fairly inexpensive and that way you can actually measure what you are giving rather than guessing.So neonatal incubators are pretty good for cats and small dogs and can usually be heated which is good for warming cold patients (even if they are not dyspnoeic).