Using regular (affordable) skin on frame building methods and simply choosing light weight wood for the frame then skinned with 8 oz nylon one can build a 16' kayak that weighs 25 lbs. This weight may not be ultralight but strenghtwise is a good compromise between an easily damaged boat and one which will stand some knocks and scrapes.

Western red cedar , southern swamp cypress , white pine or other clear light conifer wood will work fine for gunnels and stringers. My first few kayaks had coastal doug fir for the gunnels and stringers which while strong added several pounds of weight over a mostly red cedar frame. 1/4 " thick ribs can be fairly widely spaced 8-9" apart. 1/4" foam will suffice for "floor boards" where the legs rest and thicker piece for a seat.