I had the same prob. w/ the SLP lid and SLP smooth bellows. Now that I think of it, everything from my throttle body to the cold in under the bumper is SLP, you think it'd fit a little better. I trimmed the bellows and it helped a bit, I also had to trim the big rubber coupler that fits to the back of the lid. Still pops off occasionally, but I did bend one of the clips a bit, prob. my prob.

p.s. I have the SLP MAF in there, which I think only adds to the prob.