I have finished the bottom of my shield press to make the perfect curve, but now I am wondering if i have to really build the top part? I have dry tested it with some scrap plywood and I can get it to for perfectly, my solution is to use two ratchet straps.

If it works, it works. Duh. :)

Your setup should be just fine, considering I've seen people get perfectly good results with just three carefully placed logs. That said, a properly shaped top would ensure that the curvature stays exactly the desired shape, all the time, every time, without the risk of any undue warping. It's nice if you plan to use the press a lot, but not strictly necessary.

I use 5 ratchet straps with slats underneath to prevent warping. Pretty much as shown here: http://www.yeoldegaffers.com/project_shieldpress.asp