Hollow crossbeams (iako, kiato, aka) are a good way to save weight without losing strength or stiffness. In the first photo, a 5/8" (16mm) board has been clamped to locater blocks screwed to the workbench. Spacer blocking is glued to the interior surface at regular intervals and importantly where the beam crosses the gunwales of the canoe.
The top member of the beam is another 5/8" thick board and is glued and clamped to the spacer blocks.
Solid blocking at the ama end of the beam is longer to take an inserted dowel later.
1/4" (6mm) or 3/16" (4mm) plywood is glued and nailed (or stapled) to both sides of the beam. After the glue under the first layer of plywood has cured, remove the clamps and put plywood on the other side. I can't remember how much springback there was, but is was not much.
A hardwood dowel (old shovel handle) is glued into the end of the beam for a Wa'apa style connection. The end of the beam will later be finished with several wraps of fiberglass to prevent splitting.
These are the beams I've been using for several years on my Wa'apa canoe.