I spent some time taking apart my Roomba to remove some extra weight. You can see the pictures of this process here.
In the pictures, you can see my laptop with the tear-down instructions found here (click on the Discovery Breakdown link on the right). They were quite simple to follow and worked great. In the end, I removed the brush deck, side brush, cliff sensors (broken anyway), and the front bumper. I'll probably eventually remove the wheels and wheel drop sensors in order to drop some more weight. I also have a little better understanding of where I can "plug in" to the sensors for later in the project.....
I stopped by Fry's and picked up an Antec Tri-Cool 120mm case fan (about $14 after taxes). And a 9v battery for my multimeter. I picked the Tri-Cool because it had decent CFM (79 at 12v), would work at lower voltages (as low as 5v), and wasn't overly expensive. After I got my meter working, it probably didn't matter about the low voltage, turns out that the dust bin connector is putting out between 14 and 15 volts.
I tried to get the bathroom scale to work (digital), but it just gave an error after replacing the battery. So I don't know the current weight after surgery.
Now I need to figure out best to attach the 120mm fan to my (currently) 1 inch hole. I'm a little leary of making the hole exactly match the fan and directly attaching it (main concerns are about whether the bag would get caught in the fan). I'm also considering two fans vs one (at 79CFM, I'm still short of the computed 93 to 115 I need based on this calculator ). Luckily, I still have enough MDF for another base if I screw it up (on purpose -- I plan on making a "good" one once I work out all of the kinks with the original, but if I need it before then, I'm good).