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Your best bet is to taper the plenum along it's length and to lengthen the plenum past the last runner.
One issue is that the tapering is designed to equalize flow between the runners, but our runners are inherently unequal; the primaries in the middle will flow less than the significantly larger secondary ports on the sides. Also, the ports aren't uniformly distributed across the length of the motor. This is why Mazda went with the setup that they did; getting flow balance across this sort of setup is very difficult. A compound curve which narrowed the plenum by a certain volume at or just after the first secondary runner (from the TB's POV), then ran reasonably straight until it reduced again at the primary runners, and then reduced a final time into a rounded cone sometime after the last secondary.
However you do it, you have to get the air to make the turn into runners... and good luck with that! If you figure it out, then you're a better man than I, sir.
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