The skin in a monocoupe design carries a lot of the various loads. It is effectively the plate you are considering. It would only need reinforcing if their were buckling or stress points specific to that area.
Van is an aeronautical engineer with a stellar record and there are 10,000+ flying.
The thing that slows many many builders down is trying to improve a design and add "improvements"
where there is no need.
If you aren't capable (and I'm not for sure
) of doing a true structural load test or have builder/designer knowledge that exceeds Vans group, you should probably work hard on building the airplane specifically to the plans. That is the quickest and safest way to get a kit in flyable form.
Just my .02 after building a couple and watching several get the "improvement" process where none were needed.