If you're just doing the back, try to salvage your old back as a template. I used 1/2" doug fir and put 1/8" kerfs in it every 1/8" down to the outside veneer. Run the kerfs past the ends of your bends a couple inches. Grain should be horizontal to bend, if it's verticle it will just split on you. Before you try to bend it soak it in water, I layed wet rags on the outer skin and poured water in the kerfs on the inside. Soak it for a couple hours. After it was glued and stapled in place, I used bondo to fill the saw cuts and give it some rigidity. After that came the resin and mat, inside and out. The upholsterer hated me cause I glassed the crap out of it.