How about taking a shoe box cover, glueing or screwiing it to the retangle end of the wet vac hose and then mounting it to the bottom of the cutting plate with some good old duct tape?
It should improve the capture rate and work as a funnel for anything that may fall around the outside of where just the rectangle hose end would sit and you can cut a slot in the box to compensate for the area that the saw blade goes through.
Just a thought
Chance
(Don't have time, or my saw to try it yet, but wanted to throw it out there and see what sticks.)
Or, I just found this, seems someone makes a plastic version. just my luck on the invention wheel... day late many dollars short