Hi Karl,
Vents are definitely an option but as you've highlighted, water ingress could be an issue. Plus the extra work involved in.curting the bonnet and measuring etc.
One solution I've seen is bonnet spaces (also known as bonnet raisers). As you'll see in the pics it allows better cooling for the engine and obviously can be removed when you come to sell the car.
You could also add extra ducting to the engine with plastic pipework. Similar to what you would get in an induction kit. If that makes sense.
Hope I've helped :)
Cheers
Steve