small bit of cement board 6 to 9mm thick (East Blatchington BN25)

Hi,

We need to patch up a soffit and need a bit of cement fibre board that's between 6 and 9mm thick. Needs to be suitable for exterior use.

Size we need is about 175mm x 400mm. Can anyone help so we don't have to buy a whole board and just cut a bit out?

Thanks!