In article , davidlobsterpot601
@hotmail.com says...
The ones I've seen in B&Q etc are intended to be cut to size - use the
old one as a template - though TBH any bit of relatively heavy polythene
sheeting will do just as well as a purpose-bought item.


Approx the thickness of the plastic stationery wallets Poundland sell.

I know this 'cos...

--
Skipweasel - never knowingly understood.