Just finished converting a Harbor Freight tile saw to a fret slotting saw using a Stew-Mac fret slotting blade.
Sled is 3/4" birch plywood from Rockler, as are the rails and fences. I used an Irvin 140 tooth hollow ground 7 1/4" circular saw bladed for the initial cut in the sled because the Stew-Mac blade is pretty thick before it thins out for the narrow kerf for slots. I cut a couple of slots in a scrap and put a Jumbo fret in one of the slots. So far I'm pleased.