Rolf,
3mm Gaboon ply with strips of cedar for gunwales and beam landings and stringers and lightweight struss system of cedar inside to stiffen the hull as the Marmian plans don't have anything like the bulkheads specified in the AHPC MK2 and MK3 plans. I think it mentions in the MK2 and 3 plans the hulls should come out at around 20kg per hull while I think you need more like 17.5kg per hull to get down to min weight.
Knowing the weight of ply ,cedar, resin, tapes etc you can get a good idea of where you should end up before you start.
Rather than hijack this thread anymore we can discuss the exact quantities and weights offline.

Regars,
Phill

I know that the voices in my head aint real,
but they have some pretty good ideas.
There is no such thing as a quick fix and I've never had free lunch!