Just made some out of 1mm HDPE. It was easy to cut with a craft knife and rigid enough for the job. I fitted them behind the liner and it was easy to get the holes correct as you can mark them through the existing holes. I just used the 2 existing fixings and it is plenty strong enough.
Many thanks for the idea.
If I find the size and shape is wrong it will be easy to make some new ones now I have the pattern.
