My rabbit had a respiratory infection and had to spend a few days at the vet in an oxygen tent (at $200/day plus boarding!) After that, I decided to purchase the same setup my vet uses (Oxygenator Pro 5 + medium size tent) to have on hand at home, in case of emergencies.
Sure enough, the respiratory infection came back right before a holiday weekend. Rather than stress out my rabbit with a one-hour drive to the nearest emergency rabbit clinic and leave him there until the vet opened, I was able to keep him in the oxygen tent at home where I could keep an eye on him and he could stay with his bond mate.
The zippered door allowed me to leave an opening for him so he could come and go as he pleased, and he seemed to understand that being in there helped him breathe easier. He would come out to eat and use the litter box and then go back inside. By the end of the long weekend, he was doing better and didn't need to be in the tent much.
It takes time to get this because you need a prescription from your vet and my shipping took about 10 days, so it's best to purchase one in advance if you have an animal with respiratory issues and not wait until you actually need it. It may seem a little pricey, but at $200/day at the vet, it pays for itself in 6 days, and the peace of mind is priceless. I had a cat who died from respiratory disease and her final days were spent by herself in an oxygen tent at the vet when I really wanted to be there with her. Knowing one of my rabbits has a respiratory problem, I never want to go through that again.
My only caveat is, a rabbit can chew through the plastic door. If you have rabbits, you know what I mean.