I recently went to a Best Shop Jigs workshop at my favorite woodworking store, The Woodsmith, and saw this handy little dado jig. So, I decided to throw one together with scraps that I had laying around.

The idea is pretty simple. It just uses a bit of (straight) plywood/hardboard and a piano hinge.

Measure the distance from the center of your router bit to the edge of the base.
Rip a piece of plywood to that size.
Rip a second piece of plywood that's ~2x the width of the first.
Cut them to length. I chose 30" cause I happened to have a piano hinge that size, and it seemed like it'd be sufficient for most things I dado.
Clamp the 2 tightly together and join them with a piano hinge. It's a good idea to slightly offset the hinge so that when the jig is folded open, it doesn't hang over the edge of the larger piece at all. This ensures your router rides along the board and not the hinge.