Stabilized wood is waterproof. You could start with a stabilized blank or make one yourself, as Richard suggests.
My go to finish for inserts was always CA, which is waterproof, but, as you know, it has that plastic look to it. Birchwood Casey gun stock finish can produce an excellent, weather resistant finish, but requires many, many coats and is kinda pricey. One question I've always wondered about is "With non-stabilized wood, what about the ends of the wood?" Probably it's OK as it's covered in epoxy after installation. Some tropical woods are probably adequately waterproof as they are, including the rosewood already mentioned, plus cocobolo or African blackwood, or other dense and/or oily wood. All of which cost more than the Cheapskate Rod Builder Guild members are willing to pay.
How come the question comes up now, when you've done this many times in the past?