I've seen backplates on Sparkfun that can accommodate an Arduino and a small breadboard, but I have a full size breadboard. So, I made my own out of Medium Density Fiberboard (commonly abbreviated as MDF) and about $6 worth of parts and hardware.
The breadboard I have came from Radio Shack (yes, I know I paid way too much for it) and is mounted to a thin metal plate, possibly aluminum. I drilled two holes in the metal plate with a drill press at low speed.
I bought a small project box from Radio Shack to house a 9v battery which I equipped with a header from Sparkfun. I drilled holes in the bottom and the side of this, also with the drill press at low speed.
I placed the breadboard, project box, and the Arduino board on the MDF and marked and drilled all the holes. I had to go back to Home Depot to get 4-40 screws to mount the Arduino as the 8-32 screws were too big.
I put the screws in and mounted everything, using additional nuts as spacers for the Arduino board. The MDF did not take well to self tapping screws, so I ended up gluing the rubber feet on with super glue (cyanoacrylate). I scored the MDF and the rubber feet so the glue would hold.
And now the pics: (I've swapped the image hosting to ImageShack, click the thumbnails to enlarge the images)