If you're going to go for custom fit, I'd recommend getting commercially built IEMs around an audiologist mold—rather than trying to retrofit universals with a half-mold.
If you're handy with DIY, you can learn to cast. Or if you can find a shop that will reshell a particular model of universal IEMs that you know well and like, OTOH, that can be a worthwhile upgrade. (A few places used to reshell the UE TripleFis, for example.) But I generally wouldn't dink around with half-measures trying to turn universals into something they're not. They work for a lot of people; and if you're one of them, you'll like having the option.
But if you try universals and have trouble getting a comfortable seal, then I'd start shopping in earnest for customs.
If you're handy with DIY, you can learn to cast. Or if you can find a shop that will reshell a particular model of universal IEMs that you know well and like, OTOH, that can be a worthwhile upgrade. (A few places used to reshell the UE TripleFis, for example.) But I generally wouldn't dink around with half-measures trying to turn universals into something they're not. They work for a lot of people; and if you're one of them, you'll like having the option.
But if you try universals and have trouble getting a comfortable seal, then I'd start shopping in earnest for customs.