I saw this photo on another table saw question and it's basically identical to the sled that I built.
When I use the sled my hands push on the back fence to move it forward. The blade will cut the piece and continue through the back fence and emerge under the blade guard.
I have a problem where my hand will push on the blade guard and position my thumb or palm in the path of the blade. After doing some woodworking for several hours this mistake sometimes happens.
So far I've caught my mistake but I want to modify my sled to fix this problem. The only solution that I can think of is to make the blade guard longer. While this would solve the problem it makes working with the sled more difficult.
Another option would be to do something that stops the sled from moving forward far enough for the blade to emerge from under the guard, but I can't figure out how that would work.