The plans for the tender show the bends lacking foam. The curve itself adds stiffness but if this is structurally strong enough I see no need to have full core foam boards in the flat segments. The board could be replaced with an inch wide ribs of core foam along the rim of the segment and in a diagonal pattern in the center. This should be lighter, cheaper in the use of resin and be sufficient in tension and compression. A core foam board with large holes (5-10 cm) in a waffle like pattern should equal the ease of placing the segments. The disadvantage is losing a smooth flat interior face.

I found the use of this technique in building foam wings for smaller airplanes but there the holes are hot wire cut in a pattern by hand, one by one. There must be reason why such boards are not readily available. Is one of them structural?