Step 4: Make the 1/2" spacer
This box joint will be set precisely to do half-inch cuts. You could make it do wider or narrower cuts, but it will be fixed to whatever width you build it as. Any table saw that works with a dado bit will almost certainly support a half-inch wide groove. If you decide to build your jig to a different gauge, then make replace all the measurements here with the desired width.
Set the rip fence at 1/2" and the blade height at a hair above 1/2". Take your remaining 1x12, hold it vertically against the rip fence, and pass it over the blade. This will set one width of your spacer.
Next, set the height of your blade appropriate for the full thickness of board you're working with, and leave the rip fence at 1/2". Cut out the rest of your spacer.
Note: this will be tough at the end, because your smallest piece will be between the blade and the rip fence. Use a push stick, another scrap. or even spin the board around to the other side and re-set your blade. I just found that doing leaving the rip fence at 1/2" for both cuts gave me a perfect square without any thought, but it can get tricky handling such a small stick on a table saw. Be careful as always!
When you're done, you'll have a stick that is 1/2" by 1/2". Use the mitre saw to cut off two lengths of this stick, about four inches long