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I guess I don't see what a trapezoidal piece will offer 
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Hi Terry- my neighbor mentioned this when discussing bulkheads, which create a narrow and very stiff hard spot against the relatively flexible hull sides. The hull panel will tend to bend around that hard spot and create high tensile stress on outside of panel. A trapezoidal shaped piece between hull and bulkhead will create a gradual increase in stiffness as you get closer to the bulkhead, very much like a fillet, so it would reduce the stress concentration on out side of hull. However if the fillet is cabosil type material, it's probably harder and more brittle than the foam core material, so think I'd prefer the tapered foam in that case.
A cockpit floor is slightly different situation, especially if it's laid on top of a longitudinal cleat running along the side of the hull. The cleat will act more like a miniature stringer but won't be near as stiff relative to hull panel as a bulkhead. As long as there is a little space between floor and side of the hull with a large radius fillet between them, I'd think that would work fine. In either case what you're trying to do is transform a narrow very hard spot into a wider and slightly softer area so that any bending stresses around it are spread out and not concentrated in one spot. Tying the floor to the hull sides with glass across a generous fillet should do exactly that!
Denny