I have made scratch stock in the past, but Garrett Hack taught me how to make a much simpler housing and the correct way to sharpen the blade. The blade is a piece of old band saw blade and the chunk of wood that houses it has a sawn kerf in which the blade fits and a thumbscrew/threaded insert that secures it in place. Sometimes the easiest solutions are the best.
In Garrett's class, I had one of those "Aha!" moments when he explained the correct way to sharpen the blade. Once you file your profile (the one shown has a 1/4" quirk bead and a 1/4" double bead), all edges of the blade including the ends and the shape of the profile need to be sharpened. Once I understood that the edges where the flat sides meet the end of the blade, including the profiles, worked like a little scraper, sharpening was easy. The profile, the ends, and both faces must be perfect 90 degrees and razor sharp. This allows you to cut on both sides of the face. So, you can tilt the scratch stock toward and away from you and cut in both directions. You can see the curls of shavings coming off the blade and the resulting ultra-smooth profile.
On sharpening: I filed the profile shapes with round files, and used waterstones and slipstones to hone all edges. Ideally, you want a mirror finish on the faces and edges and profile.
With scratch stock, you can make any profile you can dream up and ones that can't be made with a router. This opens up a whole new world of creativity that will make your projects unique.