3D Printing

The underside of the thing looked like garbage, even with support structures in place (is that normal?).

yes, although you can always finish surfaces by hand after printing if you need to get a smooth surface, but it's not something I've done much of.

rotated the two parts so that they printed upright.

print direction and overhangs is a main consideration for me when I'm designing stuff. e.g. I always try and orientate curves / holes so they print vertically from the bed as this makes them circular rather than faceted