David,
I think if I hadn’t already sourced a Zephyr demister grille, I would have used a modern slot demister like you’re considering.
For the existing mould, I’d fill the void with expanding foam & roughly shape. A skim of filler will then give the desired shape to produce a simple mould (or left as the finished surface), from which an insert could then be produced for glassing in after the original section has been cut out.
Chris,
I got the filters from AT Power, who supplied my throttle bodies. They’re made by ITG and are the same ones used on the Cosworth DFV engines. On my throttles the base plate is clamped on the ramp pipes, as they top half with the bell mouth has a screw thread onto the lower section
Regards
Andy