I am probably the most poorly placed person to give advice on anything to do with a router at present, having yet to even sink my new machine into a piece of wood. That said, I read a huge amount (both on and offline) and came across a tip that I thought sounded useful - and might just be for this application. The only issue I can see is that it will only work when routing freehand. That said, it still sounds useful : essentially it involves creating a square router base in which one drills a central hole. One then marks one side as '0', and the others as '1', '2' and '3'. You then cut (with a table saw, or similarly accurate method) 1mm off the '1' side, 2mm from the '2' side, and 3mm from the '3' side. Attach the base to the router and make a pass with the '0' side against the fence. Rotate the router so that the '1' side sits against the fence and make a pass. The channel just got 1mm wider. Continue on, making the channel as wide as required. You could obviously make such bases with greater increments than 1mm, giving one more scope for widening the channel.