If you use something like the Etyomics, I have the ER-6s, you can have custom tips made by Westone. Get the molds done at the an audiologist and they can order them for you. I would not want to make my own, the audiologist knows how to do it correctly. The advantage is, that if they do not seal well, or you have other problems with fit, they will send them back and have them remade. I had two pairs made and they both had problems sealing, they remade them at no cost.
Ask your audiologist to use a bite block when they make the mold. The bit block will keep your mouth open during the mold process. This opens the ear canal making the resulting mold a little bigger in diameter than if your mouth was closed. When your mouth is closed while wearing the ear plugs, they will seal much better and block more sound.
I always had issues wearing ear plugs and ear phones for long periods of time. ON a 13 our day on the bike, the ear plugs would actually make my ears bleed. I can wear the molded ear plugs all day and I don't even feel them.