I thought that this might be of interest in case you are confronted with the need of hospitalizing a pigeon or other larger bird for a while.
As I did not want to spend a larger amount of money nor do I have enough space to store any cages of any size I came to following solution: I bought clear storage boxes from a DIY store with covers. The size of these boxes were 21" long, 17" wide and 11" high. A pigeon has enough space to turn and move around but not too much space to hurt himself in case of seizures and the box is not high enough to start flapping wings when the cover is closed.
The boxes are not only easy to clean but can be used what they are meant for after usage as cage: storage purposes. A cage takes place but is of no use for anything else. The boxes do not have any sharp edges so a bird having seizures cannot hurt himself. At the beginning of Pina's severe seizures I patted the box with quilt batting on the walls, simply mounted with duck tape. This helps to keep the warmth and serves as a soft isolation while bumping the head on the wall. Later on I just put a piece on the bottom beneath the towel to keep the bottom warm.
To give the bird more space above his head I used strong clamps (see pics) where I could mount the cover without giving the bird the opportunity to escape. Also the openings of the handles as well as mounting the cover on the clamps gives enough air flow. At the same time the bird is protected from drafts which is specifically important for a sick bird.
I have made the observation that our birds are feeling very well in their boxes, safe and warm. Jimi even goes back deliberately from time to time. He also loves to sit on the edge of his box and looking out of the window...
This is Pina and Willy at the beginning of their sickness: