The frame really was very easy to construct..Seriously took 15mins (using a hacksaw, drill, and pop riveter). I can furnish plans to you if you like, but it was really rather simple. Here's the basics:
I went around and purchased the beefiest bike rack I could find. My issue was that I have disc brakes on my bike, so finding a rack was abit harder, but all in all, the actual rack was rated for 25kg. REI had some nice ones for standard V-Brakes on sale (super cheap, less than $20) that looked rather strong.
The rack has thee sections: two downtubes/legs, the main rack, and two arms that affix to the braze-ons behind the seat (your rack may differ here if using V-Brakes). The two lower legs atatch to the main rack via three allen bolts, and the upper arms bolt into the brazeons, and bolt into the rack.
What I did was purchase a length of alum. angle stock (1"x1"x1/4"), and some flat bar stock (1"x1/8"). I then cut the angle stock to length, and drilled holes in it so that it would mount in the three bolts where the legs bolted to the main rack. I then cut lengths of the flat bar, and riveted it across the two pieces of angle stock. This basically gave me a rack on top of a rack.
I then drilled holes, and mounted some eye bolts to the ends of the bar stock. This gave me locations to mount bungee cords to hold everything in place.
I found it best to make the rack 2" longer than the actual length of your case, and the crossbars 2" wider than the actual length. This will do two things - #1 allow you to affix the bungee's easily, and #2 it allows the bar stock to flex abit, thereby clamping down on your case from both sides.
I also would have preferred to TIG the whole assembly, but it was done rather quickly. If you are going to rivet, use wide-headed rivets. These will give you about 250% more contact surface than regular pop rivets. Overkill ? Absolutely - but I have saved way to long to afford my camera equiptment.
I am going to rework the rack however. As I stated earlier, riding a bike with an extra 40lbs high on the back like that is like driving a tractor trailer...Really floaty. It was fine for the rail to trail and the street, but Im not sure how much fun it would have been on some single track. Im planning on mounting the case off to the side, with a tripod mount and extra water (as ballast and supply) on the other.
Feel free to contact me if you have any further questions.