+1 for an insert of some kind.
I've had several over the years. The first was a plain piece of 6mm Al. It was OK for a first go.
The second was a black plastic one with grey and red reducing inserts. It was truly awful. It is sold under a variety of brand names, and the manufacturers point out that the inserts are very slightly domed "to improve accuracy". My opinion is that that is bovine colonic waste. They are domed because it is too difficult to make plastic properly flat. It might make it very marginally better for edge moulding, but it completely screws you up for any slotting or complex edge moulding. The insert should be F-L-A-T, flat.
I then bought one from Tilgear. Al, a selection of inserts, not at all bad. The downside was that the inserts were held in by 3 tiny screws, so it was a bit of a chore to change them. Also the biggest aperture was not quite as big as the Rousseau one and there was one panel-raising bit I couldn't use with it. Not a problem any more, I don't have the panel-raising bit...
When I get what's left of my RT into the workshop, I have one of these to fit:
http://woodworkersworkshop.co.uk/epages ... MLP1613-AL
Yes it is expensive, but it is by far and away the best insert I have ever had. I shall fit it with a Router Raizer and Xtreme Xtension, as before. All three are available from Peter Sefton. I don't expect ever to have to replace this one.
S