You could try building an X-strut for each end of the treeboat, kind of an upside-down suspension bridge. You'd need to secure the struts to the branch so they won't fall out when you bounce around on the boat. Bamboo would be a good material. You could also try securing a length of bamboo on each side of the boat and fix the ends to the corners of the boat where the bottom of the struts attach. This would allow you to set it with less tension but still keep it reasonably flat.
Looking at the drawing, the struts should be set as close to vertical as possible for optimal load balancing. Ok, back to work I go.
Larger image-moss