Purchasing the extra piece of wood is the best thing. Or buy pre-made moulding in the profile and species of wood you are using. If those things don't work, back your wood to a piece of 3/4" plywood or mdf, maybe use double stick tape if you have some to stick them to a flat surface. Anchor the ply/mdf down, and another piece as your "fence" for the router to ride against and route the piece. Just be very, very careful if you try something like this. It will be dangerous. If it is a stock type profile, I'd look for the moulding already made. Safest of all methods discussed. Jim.
Home of Irish Setter Rescue of North Texas
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At a minimum, I'm Pentatoxic...but most likely, I'm a Pentaholic. There seems to be no known cure. Pentatonix, winners of The Sing Off, season 3