I have seen 3 inner spars and some even have done 4, but i did 2 at about 20 inches from teh outside spars. I put some cross braces and plan on skinning it with 1/4 baltic birch. the hatch dimensions are 61.5 inches wide by 48" in height. In your opinion am i good enough or do i need to add a few more. I ripped 2x4 into 1 1/4 widths to form the inner spars.