Back in my Formula SAE days, I had a discussion with Paul Gentilozzi (yes, that PG) and he commented that a plenum should be 110% of the engine capacity in volume.

This generalization seemed to work really well on the cars that we build subsequent to the discussion with PG as we set new records for HP and Torque using M85 (basically methanol)

The manifold that I made was basically a flat floor with 4 holes in it for the individual runners to each cylinder. Pressed and welded into the holes were basically velocity stacks that were fairly short. The 'roof' of the plenum tapered down from the inlet end to the blocked off end.

Worked great and was not all the difficult to fab.