adam i have a question for you about the design of you plenum.
this just an observation not a criticism, please don't take it the wrong way, i love the work you do and the fact you are happy to share your experiences.

ok to the question,
how are you going to fix the 2 piece shaped metal hood that you have formed to the flat oval piece that you bolt to the top of the inlet manifold.
if you are going to weld it into one solid piece, it raises the question how are you going to get a spanner on the bolts to do them up so it will bolt down to the inlet manifold.

the reason i ask this is because i have gone through the design process (and i say just the design process, never built) for a mid mounted plenum which turned out quite similar to what you are proposing to do. The biggest issue to overcome for me was how to bolt it down to the inlet manifold.
if i used a flat plate on the bottom i could not see how i could bolt it down without having some sort of removable lid on the top of the plenum to access the bolts to bolt it to the inlet manifold, or if i made it as a fully enclosed top and flat plate on the bottom for strength, i would have to make a small stand off under the flat plate and effectively extend the runners out the bottom of the flat plate and fit a flange on the end of the runners so it can be bolted to the inlet manifold form the outside of the assembly.

but that was just me and my design thoughts, just thought should share.

the other option i have seen made a vr4 engine is a kind of flat oval doughnut shape (jam filled, not ring) that was bolted through the top, right through the full depth of the plenum. but i didn't like that idea just because it limited the height/volume of the plenum.