Unhooking mat - abin bag stuffed with bubblewrap/polystyrene packing/old rags/screwed-up newspaper would do the job at little cost - although a proper unhooking mat is not that expensive, and forms a nice frame for a fish-on-the-matphoto.
Be absolutely delighted with a nice tench of 8 lb 15 oz. After all, if you had weighed it with a set of metric scales it would have weighed 4.06 kilos - so you have "broken the 4 kg barrier" as the weeklies would put it.