If you can use a sewing machine or know someone who can, then I would get two of the Xmas tree bags and splice them together. It could be a fiddly job but I believe it could be done.
As I only take my girls in the car I have made simple draw-string bags from an old tent which fits them perfectly, not entirely discreet as you have to be careful that the legs are together so don't show any human-shaped definition but it's only from the front door to the car parked at the back.
As for storage, I didn't comment because I don't hide them, I live alone with few visitors, most of which know of my girls anyway so they live in the bedroom, usually keeping the bed warm and cosy for me
