For years my saddle routing jig consisted of a piece of plywood with a hole in it and various pieces of taped down strips of this and that. The whole tedious and messy looking thing would corral the router into making a perfectly straight cut. Over the years I've had four pieces of plywood with a hole in it and somehow lost every one. I guess it doesn't look like anything, so it gets lost or put in unlikely places.
I spent to much time looking for the last one and decided it was time to make one I wouldn't lose.
A top piece designed to use with a template guide was made and considered. The idea was that the slot for the guide was small enough the placement could eyeballed very close to the correct position and then fine tuned. In practice it was to hard to see where the bit was.
A larger opening to fit the whole base was a better idea. It meant that the base didn't need to be fine tuned to the template guide and there's lots of room to see what's going on. I'm still scheming on a light that will go around collet assembly, I'm pretty sure I saw something like this for screwdrivers. Initially I used a couple of scraps with two sided tape to set the length of cut. The final design includes the knurled brass nuts with the aluminum slider.
End of story, it works really well, took a couple of hours to build and I probably won't lose this one. One other thing, it sets up a lot faster.