Gifkins dovetail sell them-
http://www.gifkins.com.au/Boxmaking%20Cutter.pdf
It's the first router bit in the photos. I have one and find it excellent at cutting rebates in boxes. I use it when the box is dovetailed (with tape for safety) and rebate the base and lid. You just have to round the corners of your lid/base for a perfect fit.
You can also get different bearing sizes that will cut 1.6mm, or the standard one cuts 5mm deep and 4mm wide.
Here is a pic
Andy