You might check out this link - Very nice work, the best inlet connection to the cowl I have seen posted.
I made my plenum cover based on a layup of the inside of the cowl. It has to be finished first or it is a tremendous amount of work to finish it smooth. I like the way Pete Howell did his. He cut some foam insulation blocks and filled within the baffle area and did a lay up on top. This way you could make the 90 perimeter flange too. Aluminum would work fine. The center hump matching the trailing shape of the spinner is not necessary, unless you have something to clear.
Personally, I would make the inlets one piece to the rings with urethane flexible ducts like DanH made his, if I were to do it again. My neoprene works great, very flexible, but having enough straight length is a design problem for the ideal inlet diffuser shape. There is not enough length on the vans cowl to waste any.
I think the most difficult area is that inlet ring to mate with the cowl. At least it would be for me.
Good luck in this project, and be sure to measure your upper and lower cowl pressures before and after to know what you have. Otherwise, it is literally shooting in the dark.