Multiple black offcuts of different shapes and sizes. Photo has 1 foot rule for scale.
The piece in the photo measures 550 x 3400 mm maximum dimensions. I have two other pieces at 440 x 720 mm max and 360 x 640 max. None of these are rectangular sadly as they are the offcuts from the corners of my pond, I have other pieces that are smaller. Obviously, all too small for use as a small pond but the butyl has many other uses such as a cover for outside waterproofing or hinge for bird boxes etc etc.
Fair Chance Policy: I’ll wait before choosing a recipient to give more people a chance to reply.