I got tired of other approaches to keeping the slots clean on my mill table so I made these slip in inserts from aluminum.
I made them so they do not go all the way to the bottom so that if there is a few "rebellious" chips in the grooves, it won't hold them up. I also made them so they sit slightly below the surface so they don't interfere when using things like a magnetic base for a dial indicator, etc.
To aid in removal since you can't grab them, I pressed in some steel discs into the 2 outer pieces. A magnet is all that is needed to lift them up/out, then the long ones are easy to grab onto.