[Dick Linn]

Don't worry about a D-shaped shaft. You can easily file or grind a flat spot where it matters.

As far as tolerances on the shaft and bearing, I'd say it's a crap-shoot. Getting pieces-parts from various sources is hit or miss and you may have to do some fitting.

Stainless may be a good idea if you want to leave this thing out in the weather. Do consider some makeshift baffles/seals to keep stuff from blowing onto/into the bearing surfaces.