Bag for mountain buggy duet?

[Valbonne09]

Hi all, does anyone have a mountain buggy duet and the bag they sell to protect it when travelling? Trying to ascertain whether it is a soft bag that would fit under the buggy in the shopping basket area until the departure gate when i can fold up buggy just before getting on the plane? Am loathe to spend the money on one but equally don't want our duet trashed by baggage handlers as we travel a fair bit. Any advice from other duet owners appreciated! Thx