Toby,
The slats work really well and the 4 of us have slept on the boat for 3 weekends so far this summer. The slats consist of multiple wooden planks, say each about 2" wide, linked by 2 strips of webbing material. When not in use the slats roll-up (and live in a locker). When used the wooden slats sit on top of the wooden trim that runs along the inside of the cockpit benches. Finally, there are stainless clips on bungee cord at each end of the length of slats that attach to deck clips (a pair either side of the outboard and another pair just outside the companionway) to hold the slats in place. The slats wouldn't be hard to make but probably time consuming because the curved shape of the cockpit means every plank has a different length.
If you're interested then I'll post a picture. If you want to see the slats then I'm only down the cost from you in Dorset.
Jonathan