You'll find these commercially available, but they didn't have the configuration I needed, so I made my own.  (If I have understood how this site uses graphics) The attached (or displayed) shroud is set over an exhaust intake that transitions to the 4" duct below the bench.  With a miter saw, you really don't capture much by direct tool connection, so an "area exhaust" is the best you can do.  This one is about 90% effective (a MS is a very energetic device), but that's about twice as good as bouncing the dust off the back wall and hoping....  This configuration also works with my spindle sander and router table.  It's a bit jury rigged (plastic cloth taped to curved spines, separated by 1/4" rods), but it really works well.  The clear plastic (Amazon) cost about $10 and allows lots of light on the workpieces.

Nice build!

well done Pete...

beats the snot out of my plastic storage bin...

Very nice work!

Nice design and it works to keep the dust down.