I used this jig to do all of the dados on my Electronics Workbench, and I also used it to make my End Mill Tray.

The knobs on the right hand side allow the width of the slot to be adjusted. The knob on the left is used to tighten the clamp. Bigger...

The clamp is comprised of two layers of plywood with a machined block of Delrin in the middle. The delrin has a "half nut" machined into the side. I did this by drilling and tapping a hole, and then machining away half way through the hole. Bigger...

Here's the bottom of the clamp. There's a 1/4" dowel pin which rides in a slot the runs the full length of the jig. The dowel acts as a pivot, allowing th half-nut to be moved away from the allthread and allowing the clamp to be quickly positioned. As the clamping portion (left side of this picture) is tightened, it forces the half-nut tigher into the threads. Bigger...

Here's a picture of the router positioned in the jig. It happens to be setup for a 1/2" dado, so the router touches on both sides. For wider dadoes, you run the router up one side and down the other. In this particular photo, I was creating some rounded dados for an end mill tray. Bigger...