Ain't nothing wrong with overbuilding !! I know that in 'i-beam',floor joists or roof rafters that 7/16th OSB on edge with a 2x3 top/bottom holding it, go WAY farther on spans than I figure it should/would....but the engineers say yep.On your big arch I can see where it would be too much waste etc....but the average guy supporting his 'stock' under mast support would find great additional strength by simply laminating a nice piece of mahogany 3/8th ply to each (for/aft) side. Way easier than the steel bolt on in the manual and looks better too.