About 8~9 years ago Bridge City Tools came out with a fine little jig
that enabled you to cut perfect fitting dados with just about any width
blade or router bit. That is as long as the blade or bit cuts narrower
than the width of the dado.

I bought that expensive little jig and have had great success when using it.

Seems there is a shop made version now and probably the absolute
simplest that I have seen.

Take a look!

https://www.youtube.com/watch?v=fNZWlLPw0HI