Happy to have finished the cushions today. The inserts were about $8 each, and the fabric all came from my stash. I used the zips I have been collecting from op shops at $1 a piece and scraps of my quilter's batting. There is a very ugly piece of fabric behind the batting as a liner for the cushions.
In most cases I used both the old insert and the new one in the cushion -to bulk it up. I am only 5'1" tall so I often need a cushion on a chair which is deeper in the seat than I would prefer.
I realised after I tried to attach a zip between the two quilted panels front and back of the first cushion, that this was not going to work with the width of a walking foot, so I made a placket for each of the zips and then attached the placket to the cushion covers with my walking foot. The result is that each side is pretty!