I'm building a custom bike from a 1980 Sportster. I make products for custom bikes, but this is something that I did as a one-off for my personal project. It came out so well that I'm selling pre-cut kits, but there if you can cut and weld there is no reason you can't make one like this from scratch.

The complete build up of this project is already on my blog, you can see it there.

The basics are:

First I drew up the concept in CAD:

After tweaking a number of things I had the parts laser cut, cleaned up all of the edges and surfaces by sanding through 320 grit since I'm planning to have this chromed:

Next I welded up the box itself, I used a cheap machinist's angle plate to keep the sides square: