i built the previa's roof rails out of 2x2 lumber painted black with some nice outdoor paint, that was five years ago and it's still holding up. the steel roof rack i attached to the wooden rails is rusting where the powder coat has chipped and it's leaving ugly stains on the van!

lots of people DIY-ing roof racks/rails out of unistrut, or 80/20 aluminum extrusions, or various types of metal tubing, but no one is just using lumber. if i use lumber, i can drill into it wherever i want, i can use screws instead of drilling holes for nuts and bolts, and i don't have to worry about rust stains dripping off the rails where the paint inevitably chips