I needed riving knife slots in homemade inserts and scratched my head on the same problem. The solution I figured out is:
RIVING KNIFE SLOT CUTTING PROCEDURE
0. Unplug the saw as appropriate for safety.
CREATE THE ZERO CLEARANCE SLOT FIRST:
1. Remove or lower the riving knife out of the way.
2. Lower the blade below the table.
3. Place the new insert over the hole.
4. Clamp a scrap 2x4 on both ends across the saw table to hold down the new insert.
NOTE: You can use the rip fence, but be sure it doesn't cover where the slot will be.
5. Power on the saw, and raise the blade to cut the zero clearance blade slot in the new insert.
6. Lower the blade completely.
EXTEND THE ZERO CLEARANCE SLOT FOR THE RIVING KNIFE:
7. Remove the new insert.
8. Install an old insert. Any old insert will do.
9. Raise the blade above the table just a little bit.
10. Take the new insert and place on top of the old insert, with the slot over the slightly raised blade. 11. Move the new insert forward with the blade still engaged in the slot.
12. Clamp the new insert down with the 2x4.
13. Lower the blade.
14. Power on the saw and raise the blade to extend the slot backwards in alignment with the existing zero clearance slot.
15. Install the riving knife and test to see if it can raise it through the extended slot.
16. If yes, you're done. If no, then repeat steps 9-15 until yes.
Note: On some saws, the slot for the riving knife cuts through the back of the insert. That's how they are.
Here is an earlier solution that I didn't like as well.
1a. Create the zero clearance slot as described in steps 1-6 above.
2a. Remove the new insert.
3a. Install an old insert. Any old insert will do.
4a. Raise the blade above the table just a little bit.
5a. Take the new insert and place on top of the old insert, with the slot over the slightly raised blade.
6a. Bring the rip fence so that it touches the right side of the new insert.
7a. Carefully make a stopped rip cut through the back of the new insert, to create the riving knife slot. Done.