My table saw came with a 2 part aluminium insert that was slow to remove and left a large gap around the blade. I had some perspex the correct thickness so made a new insert. Mostly simple, trace the shape and cut it out but I did need to route a little recess for the arbour when the blade is at full height.
Once I had thing set up I made a few spares.
Very easy to cut the slot. I removed the riving knife, used the fence to hold the insert down and raised the blade through the insert. Once it was full height I then flipped the insert front to back and did the same procedure, this extended the slot toward the back making room for the riving knife. A little file work to smooth it out and i was done.