I came here to see how you guys build them.

Mostly with wood/glass, although the range covers skin on frame. Now that would be trick, strip floats. You're initial suggestion of a few layers of kevlar and s-glass sounds quite substantial, would this be a foam core hull or solid foam like a surfboard? I've seen some ultralights with inflatable floats or looking like a little zodiac, is that the size of ultralight?