I ran RPM carriers once, I hated them. The pivot balls would pop out of them worst than the old style Traxxas carriers. And I don't recall that they use a 8X16 outside bearing. I think it's a 15 mm OD bearing. But the plastic of RPM parts is so soft, you could probably press a 16 mm bearing in that 15 mm hole.
I thought about the Tekno carriers, they use a 16 mm OD outer bearing but I don't think there's any way of making a 21 mm OD inner bearing in them.