The catalog company Brigade Quartermaster once sold U.S Army ponchos of urethane coated ripstop and a BQ proprietary liner with quilted Thinsulate that snapped inside the poncho on the OEM poncho snaps. It had a special drawstring affair for closing the top and bottom.
Cal BQ and see if they still sell it. Remember, the poncho doesn't breathe and this is merely an expediant "sleeping bag" for warm weather. But it's better than no liner. (The Army Rangers used this setup but remember, their unofficial motto is, "Travel light, freeze at night." :)