Thought I should update this post as I actually finished my crossbars a while back. I ended up using square section steel pipe and square Ubolts. I sanded the bars and then painted them and the mounting hardware with black textured rustoleum paint. I filled the ends with silicon sealant and the pushed squares cut from a black rubber pipe fitting into them.
Unfortunately, the silicon was white because that's all i had available, so that kind of messes up the look a bit, but the bars are strong and tough, I've already carried several heavy items on it and it's held up fantastically.
I was also able to adapt the bolt holes in the bars so I could put the bolts open-side down, thus making it easier to carry wider items without risk of punching bolt shaped holes into them.
Here are some pictures:
I may still buy some proper end caps for them, but for now the silicon + rubber squares do the trick.
One disadvantage of the square bars: They "moan" at highway speeds. I've noticed a low frequency wind noise that shows up at higher speeds ever since I put them on. I suspect they aren't helping gas mileage either. I may have to add a wind deflector of some kind. I suppose I could DIY up something like that too. I'll have to see what I can do with it.