I've got this fantastic ingenious way of keeping mine from closing its called a hand, I've got a spare one in case it gets injured
or if you drill,cut and bend £5 for a 1m http://www.ebay.co.uk/itm/Mild-Stee...hash=item3f54806d51:m:mRSrqj64JXCS5EyAyk6MfFw
Nothing so fancy here, a bit of wood instead of the steel support thing screwed onto the inside of the bed, I'd estimate about 40mm x 15 x 10 inches long. I did have a picture but seem to have deleted it, can provide one of those tomorrow and precise measurements if need be. I can't take any credit because it was in the bus when I bought it.
I use a folding chair in its bag. Wedges nicely in the frame under the seat and across to the front of the rear facing seat opposite. I would worry about one of those simple metal props at one end because with 5 inch firm cushions there is a lot of force pushing the seat back down.
I'm using a fold up wind break in a bag at the moment but want something a bit more permanent. As you say it would need to be something sturdy.
ive got a piece of wood too 38 x25 with top cut at an angle which sits behind seat frame to stop it sliding out , works a treat and doesnt really take up that much room and can be used for other things if needed