i had a brain-wave . . .
i made a box shaped "spring" to fit inside the bellows
to help retain the shape of the inner material to that
of the shape of the bellows or in plain english,
to stop it from sagging! 
it works great, took about 30 minutes.
The images show below how the inside of the bellows
is kept "square" both folded and in the open position
but the outside when folded does not retail its original
fold. the image #02 shows where the material had been sagging.
I taped the ends of the wire spring to prevent them
wearing the material.
It'll do for the short term.