WANTED: Hardboard for hole in cupboard (RG26 Baughurst)

Hardboard for hole in cupboard
WANTED
Hardboard for hole in cupboard
RG26 Baughurst
DESCRIPTION
I’m looking for a piece of 5mm thick hardwood (preferably white one side but I can paint it if not) approx 30cm x 30cm, or white plastic, to cover a hole in the back of a kitchen cupboard. Or ideas.