I've had the Covercraft UVS100 for my other vehicles. It is a rigid cardboard type structure that folds accordion style, as noted above. Because it is rigid, it will not sag when in use and you do have to be mindful not to bend it while not in use. It is not flimsy. Ours have held up well with use. You do have to find a place for it when not in use. We place it on the floor of the back seat, if we are frequently using it. Otherwise, it is the trunk.
I am able to reuse my existing UVS100 (for an Accord) on the Passport. Not 100% coverage, but good enough.
If you prefer to just toss the sunshade in the back seat, you may wish to consider the softer/flexible ones.