Make a test mortise with the machine, then measure the length, width, and depth of the hole. Typically whatever the wood is I'm using for my project I'll rip some scrap to the thickness of the mortise, then rip that to the width. Or width first then thickness, whichever is most convenient. Then I'll either use a 1/8" round-over bit in my router table to knock off the corners to fit the mortise. Finally cut them a tad short so the don't completely bottom in the mortise. If necessary, fine tune with a block plane. I'll try to fine tune before cutting them to length so while they're still in a long strip. Let me know if any of this doesn't make sense.