My only advice is to minimize the curves anywhere possible. I have tried this twice. Both times I used a tube salvaged from a junk HVAC unit. Looks like copper but feels a lot harder than I would expect copper to be. It has about a .025 inner diameter. I'm sure stainless would be better if you can get the edges smooth.
The first one, I routed strait along the neck and ran the tube about "3/8 from the edge of the neck. I made maybe a "1 radius turn before the 5th then up at a pretty steep angle. It was prone to stick and drag. Hard to tune, broken strings... usually broke right where the string came out of the turn going toward the tuner, not coming out of the tube and over the fret where you would expect.
Second one I made a slow swinging curve with almost no angle into the fret-board (routing up into the fretboard) from the bottom. same tubing. Works pretty good.
Was going to suggest having a look at this topic but noticed you are the OP of that one as well. In it is a good photo of Rudy's exit on the peghead and that he suggest that there should only be one bend. banjohangout.org/topic/350392
I'll be happy to send you a length if you'd like, just PM me your address. I have used it on four banjos to date. so I have used two lengths and have eight left, since each one is enough for two banjos. It is not prone to kink easily, I just bend it by hand and have not had it kink yet. The OD is about 1/16" and that is the size drill bit I use to make the holes at either end. Here's a link to an old page with a couple of close up pictures of the ends of the tube on my first one. I now run the tube a little lower in the neck, but either way works fine. I don't think the wall thickness is critical, I tend to think that having a little bit of extra room in the tube for the string to slide more easily is probably a good thing, but I don't know.