Who says I'm not already doing this nozzle design and just haven't showed the other side of the plate?
It's not a perfect nozzle inlet in the ASME sense, but it's not a straight cut either.
The reason why the solid metal plate is there is to provide a strong bracing pressure against the tops of the fins. With the very thin base-plates I'm using this is an essential part of the design. I investigated the inlaid nozzle plate idea, but for the low volumes of blocks I'm making, the tooling costs required to setup and stamp out some plates or make up a plastic mount, it was cheaper to just get the copper plate made up as is.