Alternatively, you might be able to fit bonnet extenders (can't think of the correct) name which allow you to fasten bonnets down into place, but with approx an inch of air gap at the rear. That lets quite a lot of hot air out of the engine bay, to the outside of the cockpit area.
Techniques might be a supplier. They're usually found on +8s as the footwells can get very hot in UK summer...
Cheers,
