I have, strangely, a P2 bonnet but the 'air intake' wasn't functional. It had a plastic 'egg crate' grill on the front with a solid grp wall behind so no air would flow through it. I've seen a number of P2 bonnets with 'modified' air intakes (like the one on the Petrolicious car) that would allow air to flow through. I've done the same because my plastic egg crate was damaged so I've removed it and opened up the entry point. Hopefully, it will add a little extra to the cooling.
I have seen a lot of Ph2 over the years with or without the vent cut out and I am sure it was all down to getting them out the door more than anything else.
IMHO there is no real issue with getting air into the car so long as you have a well fitting air intake from the nose to the rad (very important) its getting the air out that seems to be the problem due to the ground clearance etc, especially at higher speeds. Although mine only misbehaves in slow traffic, over 20 mph its fine.
The Ph2 vent would help when parked as it would allow the hot air out. The vents at the back edge of the bonnet are great until your engine start to become a little asthmatic, then you get a film of oil on the screen
I put a piece of sheet timber material behind the air intake and drew round it and cut it out.
Then cut another sheet to the outer dims of the rad.
measured the distance between both in the car and screwed blocks of wood between the two templates. this creates the front and back of the air intake.
Then I filled the gap in between with blocks of polystyrene and craved it back to match both template shapes. This is a very difficult shape to create any other way IMHO as one is rounded and one is square and they are both very close to one another when you ahve to blend the two!
Wrapped it in tin foil, coated up in GRP.
when set, cut it in half, pop it off and added flanges so it overlapped for fixing.
Put neoprene around the front edge and pushed it into the nose, wrapped neoprene on the back edge which touches the rad.
My rad is only held in with long timber tappers that slide down the existing steel rad supports. this is so when you slide them down they slowly push the rad forward and presses it against the neoprene/air intake and holds everything together.....