The crosscut sled is zero clearance all by itself.
It's hard to tell from the pic on Bill Fitz's site but I build them with a higher fence for clamping on auxiliary fences for cutting tenons.
I also attach a box to the front-center of the fence which guards the blade as it comes through when you push the fence to the limit of it's travel.
And finally, put a fence on the back of the sled. Then you can shim the workpiece tightly between front and rear fences to avoid slippage.

Making a zero clearance insert:
Use the stock plate as a template and cut 1/2" plywood to the shape.
Now run 1/2" #10 screws into the bottom of the insert so the heads sit on the flanges that hold it and adjust the screws to make the insert level with the table top.
Remove the 10" blade and replace with a 6" or 7".
Lower the blade all the way down below the level of the table (you can't do that with the stock 10")
Lock the fence CAREFULLY over as much of the insert as possible to hold it down, taking care not to cut the fence as you raise the blade with it spinning. It will cut a long enough kerf so you can repeat the process with the 10" blade to get the kerf full-size.
Make inserts for all your blades, thin kerf, regular and dado sizes.

• 80 tooth triple chip for veneer plywood $39.95