Still, I thought the idea might help someone out there...
Note: I originally used it as pictured here, but found it to be more comfortable when I turned it around. It works much better in that position - you lift your toes UP slightly to click with the heel of your foot. The mouse as depicted, should be turned so the triangle (in my picture) is pointing towards you, and the hinges are toward your toes. I also put a piece of 2x8 under my feet (with only my heels resting on the mouse) so my feet are flat and fully supported in the resting position. The concept needs a bit more work, the ideal solution would keep your feet flat on the floor.
This is the basic design: [NOTE: The flip flops are not part of the device, they are just there for the picture, to illustrate where the feet go]
SEE UPDATE ABOVE, this picture is no longer appropriate, imagine the flip flops turned around instead.
It was built completely from scrap that happened to be in my workshop. Basically all it consists of are an old Microsoft roller mouse (which are notoriously reliable by the way, and can probably be found in thrift stores if you don't have one laying around), some scrap pieces of wood, hinges (can be taken from a trashed door, or you can use smaller hinges which are pretty cheap to buy from a hardware store), and some dual lock (which is basically the same as Velcro, only stronger - but you can also use Velcro which is cheaper) and some springs.
The dual lock / Velcro, allows me to move the mouse slightly forward or backward, which in turn adjusts the petal sensitivity. If it is moved away from the pedal, it requires more force to click the mouse, if it is moved towards the pedal, less force is required to click the mouse. This is an important feature, because you have to adjust it just right so that resting your foot on the pedal does not click the mouse accidentally.
Note that the ball should be removed from the mouse. This is because you will be using another device to do your cursor control, you don't want your foot clicker to move the cursor (which is why a roller ball mouse is preferred over a laser mouse - you can take the roller ball out of the former and ensure it does not move the cursor).
An interesting side benefit of using this setup is that it FORCES you to maintain good posture while at the computer (i.e. feet and legs in the correct position). This takes some getting used to, but when all is said and done, should be for the best.
The springs are about 1.25 inches long and 5/8 inch in diameter. These springs are really tight, I don't know where I got them, but the description is on the bag. You can probably buy similar springs from a hardware store. I drilled a 5/8 inch hole in those little round pieces of wood I happened to have in my scrap pile - the springs fit in perfectly, and the extra wood just holds them in place.