I had a set of custom ear molds that never sealed as good as the foam tips. I never got a good suction from the ear molds, and even the slightest bit of air between the mold and ear canal amplified outside noise.
At the time, I was on an emb-145 which is known to be super loud.
I spent money at an audiologist to make the mold, then like $150 (which I was refunded, eventually) on the ear pieces. I’m on my second set of CA’s and I still use the foam tips.