Why do you want to do this? As they say "good question". Like the saw kerf on a table saw you make a zero clearance insert for the table (zci). On a miter say you can do the same basic thing either by making one or buying one. But the cut also likes to splinter on the fence size of the cut. So you want to make something like this to fix that issue.
Another thing that comes up is when you want to cut smaller pieces that can't bridge the gap between the parts on your miter saw. Or in general are too small to be done safely with the existing fences. With this you still have to take all safety issues into account but at least the small parts can be managed better. If it feels unsafe don't do it!
So now that you know the why, how about a look at what I did:
As a basic method to make something like this I just cut some 1/2" baltic birch plywood and traced the existing fences including the hole locations. Some fences may not have mounting holes but it shouldn't be too hard to make some?? Then it was just a matter of cutting them out and making sure they fit as expected. If the tops of your existing fences slide and a lot will, you may have to account for that to allow them to slide with the extra parts on.
I still have to sand mine and round some edges but I'm done for the day on this project so that will be another day.