Thin Rigid Board?

Owain_Moneysaver

Signwriters will cut correx or foamex (although they can be cut carefully with several passes of a stanley knife) and they might even have a used sign they've taken down which you could have for a donation to the staff biscuit fund.

LadyDee

Just an update. I bought a piece of hardboard in Homebase - quite cheap and they have reinstated their cutting service.

Thanks for all your suggestions.