Pontus, I got my wood ("furu", not plywood) from the local building supply store, Beijer. Bauhaus has similar wood but in shorter lengths. Beijer has it up to 5M long and with very few and small knots. Really nice. You can choose whether to buy it cut and planed to approximate sizes or buy a say 18mm plank and rip it to size with a table saw (Chinese at Bauhaus for about 1600 SEK) or circular saw with guide. I use "trallskruvar" to hold pieces together while the epoxy hardens. To save weight you can remove them and fill the holes. I kept them only on the gunwale as I used 20x20mm oak for that and I wasn't sure how well the epoxy would stick to that. But so far no detachments.
Peter