At 165ktas we measured about 14.25 inH2O which is about .51 psi. The upper plenum area is about (very rough mental measurement here) 360 square inches. This results in a force of about 186lbs exerted upward on the upper cowling.
Average Lycoming plenum lid is about 32x24=768 sq in, so force is about 391 lbs. That's just for the area of the baffle walls. It must also withstand an additional ~0.25 psi over the balance of its area. The cowl puffs up quite a bit in flight.
Back to the plenum lid. Remember, 391 lbs is at cruise speed for a average RV. Q rises with airspeed. This one is withstanding ~550 lbs at 200 knots. If you compare with older photos you'll note I found it necessary to reinforce the attach methods:
I often see lids with no fasteners at all across the front behind the spinner, and widely spaced fasteners elsewhere. We've all looked out the windshield and seen how the cowl edge blows open between fasteners in flight. That's what happens with 0.25 psi. Consider what the edge of a poorly designed or fastened plenum lid is doing at twice or three times the pressure. Moral of the story? The mere presence of a plenum lid does not guarantee great sealing.