Approx 100mm x 60mm, hole in center, holes at each corner.

needs to be fairly stiff as its replacing a sheet of steel but doent take any significant load.

Obviously I’l take proper measurements as and when.

Oh, how much?
looks a bit like the spider shaped plate at the bottom of the photo (witht he 4 screw things dropping down (the 4 outside holes)

http://www.easycomposites.co.uk/Products/Pre-Preg-Carbon-Fibre-Sheet.aspx

Assuming it’s quasi-isotropic layup it will have about the same properties as aluminium (70 Gpa) so you can work out how thick you need compared to steel (200 GPa). ie if you steel bit is 0.6mm thick then you need about 2mm thick carbon.

Would offer to cut you a bit of scrap to the size (rectangle, nothing fancy) but it isn’t nice pretty woven material.