I would begin with a "drop base" A/C bottom to take advantage of the radius shape around the opening. Extend it in all directions, especially rear (18 gauge aluminum + pop rivets, caulk it when done). What you want as result is large cross-sectional area right over the air horn. IIRC it's about 5.125" across for almost 21 In.2? You want double that (7.25" circle, 8.5" X 6.5" oval, etc. with as much height as will clear the firewall. Big internal volume (no such thing as too big) insures a large reserve of still air (intake velocity is traded for pressure), in which the venturis draw as if they were in free air.
Any form of screen: big restriction.