Whoops, in the picture I didn't solder the tabs to the copper bus bar but screwed them to it. The insulator is between the bus bar and the case. I did make another straight LM317 PSU with the regulator soldered to the bus bar. I don't have a picture though.
That is a large box, I'm going as small as i can. Given that I'd have to pot the thing anyway and that metal cases are much more expensive it looks cheaper to get an open potting box and just use cheap small heatsinks leaving the fins to stick out of the potting compound. I think you can get aluminium cases with fins on them that are open one side and you just screw any TO220 packages to the bottom (fined end) and pot the things up leaving a flat bottom but i bet they are not cheap and I've never seen them for sale, but have seen them in automotive stuff.
How big is the box?
Potting it in a normal plastic box with the heatsink sticking out does sound like a good idea. You can also buy special potting which is more thermally conductive but it's probably more expensive.
If you sand it or get it painted matte black it will dissipate a lot more heat by itself (no fan) than if you just leave it as shiny metal.
Yes that's what I did, although the finish wasn't as good as I'd hoped for. I probably didn't sand it enough.