Molding in a hood scoop

I have a fiberglass six pack hood scoop on the way for my Duster and although I have been in the auto body industry for some time now,Im still not afraid to ask for opinions so here it goes.Ive molded in a couple of these in my life.One was a cowl induction on a Chevy s10,turned out great but years later saw the truck for sale at a used car lot.SO I went to check it out and saw it was barely starting to crack( I guess it never cracked for the original owner even though I told him I couldnt guarantee it would never crack)The other car was a snorkel hood scoop I did on my brother 74 Duster and that one lasted for 3 years but started to develop a small hairline crack up front.Ive even molded in ground effects for peoples ricers(yuk,but I needed the money,lol)and I even told them there was no guarantee it would never crack.On the hoods,I stripped the hoods down to metal first,made sure the scoop was lined up correctly,revited on,and then I roughed up the surrounding edge of the scoop so the filler would bite.I then spread kittyhair around it,sanded it until it blended in,skimmed it with a light coat of Evercoat filler,then finished it with some glaze putty ,and primered it.
I want to know what some of you use or what methods because I plan on keeping the car for along time and do not want it cracking so soon,or ever.Im thinking slamming the hood too hard contributes to cracking,and the heat of the sun and the cold.
Im open to advice,thanks!:cheers: