Mike Stocklin in Faith, SD regularly makes Siouxan style horse bows in the 70# plus range using rattan and sinew.
Saw your rattan down the middle, flip it arsey-varsey so the cut side is out and start tillering from the back. Once you have it floor tillered, string it and finish tillering with the string still on it to show the flat spots. If you have access to a belt sander and a bandsaw there is no reason for you to take more than an hour to have your bow finished and shooting! You can leave it unfinished, or sinew back, or rawhide back, back with canvas, silk, paper bag, what ever you wish. This stuff is seriously tuff!