Costco in the UK sell a rubber mat 80 X 100cm which is a near perfect fit and covers the spare wheel cover & battery cover.
It is heavy duty and has a lip around the edge which will retain drips from wet clothing, boots etc. It also stops things sliding around.
I have bought 3!
Costco item is 154396
Bar code number 5014 2063 70374
I have not been able to find it on their website but it doesn't work very well.