Slippery, the ideal design might be to create a sleeve to slide the molding into with a stop block. The top of the sleeve would be cut to the profile you need, then trim using a pattern following bit. This is a different type of flush cutting bit, the bearing is on the collet side of the bit instead of the far end like a flush cutting trim bit. I am guessing you make a slight relief in the back side of your cut to allow some adjustment? Use wedges and some 1/4" plywood to make a "ramp" at an angle so your router is fully supported at whatever angle you need for relief. If you design your jig with a second pocket you can make guide slots for using a hand file to define any sharp corner details. Another thought might be to use Rotozip bits which could self guide on the smooth part of the shaft. Either way i would use a hardwood edge for your guide, or perhaps some UHMW plastic?(available at Woodcraft) Once you have this jig worked out you can have an unskilled laborer make your cuts freeing you for other things.