Aside -- Traveling alone, I *might* be able to fit the boxed stormdoor
> fully inside the Prius. The front seatbacks recline to the horizontal,
> creating a clear span from the liftgate to the dashboard. (See the
> owner's manual under "Seats" for the how-to.) I'll check that
> possibility and post the outcome here. A major consideration will be
> the extent to which I can secure the box against movement in an
> emergency.
Alas, it is not to be. The seatbacks-down configuration does open a space long enough and high enough, but the resulting gap between the driver's seatback and the rightside B pillar is too narrow for the boxed stormdoor. Also I found no dependable anchor for securing the package at the leading end.
The undulating "floor" of the created space compromises the space's utility. One would be hard-pressed to stretch out there comfortably in a sleeping bag, for example.
Na'theless, there likely are cargos for which the space would be appropriate, and any possibility is better than none.
Brent "Only a poor fisherman curses the river he fishes in." -- Carl Sandburg in "The People, Yes"