I've been playing with this idea - firstly in my head, while grouting tiles! Here's a first shot which shows my 18-24" depth was excessive, as it's 12" deep when folded.
This is a bit too long at nearly 8ft - I'm assuming an interior height of 12" is needed over a 72" length, which means that the 'tent' has to be around 90" long as the ends taper to nothing - but that would be good overnight storage space.
This design depends very much on how solid you would want it to be - I think this would be the perfect application for some super-light techniques, like making your own wafer-thin fiberglass and bonding it either side of foam sheets to make home-made sandwich panels - this could easily cut the weight by half or more, though you could kick a hole through it if you tried.
The first practical problem is finding a door latch that is no thicker than the door on either side, otherwise the side panels won't fold right down.
Andrew