Site traffic seems to come and go on some of these forums probably no real explanation for these trends - I've always tried to post in to
help others even although I currently do not now have one of these cars.
The circular piece of hardboard really did sort out that late 2009
Ibiza and all the earlier cars I fitted into to - though that late 2009
Ibiza did not have any stiffening material under the boot mat. A pity the proper fix to make that mat stiffer has turned out to be a failure, my hardboard - which looks to be the same material that is under your mat, only completely covered the spare wheel area and kept the boot floor flat without any creaking etc,
Edit:- a possible fix might be to fit some padding strips round the area directly above the step at the edge of the spare wheel well?