The NBRSA rules specifically prohibit metallic materials being used in the construction of the rear bag. The bottom inch of the bag is exempted from the deflection (1/4", horizontal with finger pressure) rule. The problem with a rigid flat base is that not all benches are perfectly flat. I think that the oversized Otto rings that Edgewood offers combined with a rigid bag bottom may be a pretty good combination. Another option, Protektor Model will make custom Otto rings to your specifications for a fair price.
Personally, I believe that there differences in stocks, and perhaps bedding that account for differences in how rifles perform with hard front bags. I have thought of coming up with an oversized front bag, that would not fit in a normal top. I have one of the original Hoehn windage tops that could easily have an oversized bag plate fitted to it, as long as there was a spacer to let it clear the windage adjustment knob.