An advice for those who are going to make this themselves: Mortise the boards into the frame while the frame is still green. The hazel become remarkably hard to work after it has dried, mortising properly is next to impossible. What I did was to plane the surface down to make it flat and narrower, so that the cordage grips binds the boards more securely to the frame.
Tie the boards down very hard. Preferably with wet cordage, so that it will tighten up when drying and not loosen again when it becomes wet. I may have to redo just that, if it becomes a problem.
The straps are tied under the boards and the frame is now finished.