[sensible]

I have bought the inflatable bag but not fitted it yet. I had researched how to do it. You also need a plastic A4 folder cover and put that between the springs and the bladder. That way, when you inflate the bladder it pushes against the plastic folder cover and inflates into your back rather than inflating through the springs...if you get what I mean.

See these pictures. The bladder would go behind it like it does with the electrically operated ones. The flaps on the card hold it in place on the springs. I have just got to work out how to attach the bladder to the plastic card so that it doesn't move. There is a rim around the bladder with holes in that could be used to secure it to the card by some means. You would only secure one side though (like the original electric one is only secured to the card at the top and hangs down)