Its the engine heat that causes them to sink/crack. My friend has a 5 Turbo with an Evo vent and he has had the same prob. Best thing to do is throw the bonnet away and get one of those old Celica GT4 vents (like a Scooby vent only slightly smaller and alot deeper) they look alot better on a Corsa :)
Evo vents just cause problems and IMO the don't look as good as some of the other vents available.

if it is done properly it wont crack even with the heat from the engin, it is only slaming it maybe.

same thing is happenin with my bonnet vent so i took my car back and he admits it was his fault but what he is gonna do is pre heat the vent beofre paintin it and then it should be ok from the heat from the engine. the cracks could of come from slammin the bonnet or the heat from the engine, just got to be carefull.

well warped's idea was good try it but really u will be going back another 6 months again with the same problem it happens to all bonnet scoops u need to shut bonnet slowly an lift it supporting both ends cos this avoids cracks an so on too

best thing is to start again,glue and rivet to bonnet then fibreglass matting to blend it in and finish in top stop filler

if it was a pro job that u payed for then take it back and shout about ut consumor rights and his product not being fit for the purpose it was intended :)

HI I HAD AN EVO 6 BONNET PUT ON MY CAR A FEW MONTHS AGO. IT WAS SEAMLESS BUT AFTER A FEW MONTHS IT CRACKED AN PROGRESSIVELY GOT WORSE. ANYBODY GOT ANY ADVIVE TO OVERCOME THIS AND WHAT MATERIALS ARE BEST TO USE? THANKS