Yea I do the same as Gerry(sort of) i had some polythene sheet left over from my UFH job so used that, nothing fancy folded it in half and sealed the other edges with tape, get a waterproof grade as not sure if some tapes are air permiable.
Should be able to see it in the photo.If you doing a lot of veneering then go for a set up like Gerry's.
Mike