Go to your local caravan supplies shop, or Ebay. Purchase a length of bungee cord - say 2m. Where the slots are on the seat frame, run a drill through to suit your bungee cord. Make up the wire hooks for the front from old fencing wire or similar. Tie a knot in the end of your bungee cord, then run it through the holes and round the ends of the wires until you reach the far end of the seat. Tie another knot and cut off the surplus bungee cord. If you find the seat a bit slack, tighten up the bungee cord, make a new knot at the end and cut off the surplus.
I first used this system on the Cammy where it worked with no problems for 15,000 miles plus. The system is currently in use on my 4G and after 2500 miles I am seriously thinking of tightening the bungee cord slightly. It may not be original but it works, is very cheap and nobody spots it.
36 4G, 37 VH, 53 ex ISDT KHA (project), 54 KH(A), Healey 1000/4 (project)
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist once more - but not Machine Registrar.