You might be able to do it with the router like in the diagram. This might work better if the slot isn't exactly square to the edge of the wood. With the router unplugged, use it and the bit to align the fence to the edge of the baseplate at each end but use a shim similar to the extra width you need (maybe cereal box cardboard or something). You might need to turn the bit so the cutters fill the slot (no play).
When you route the slot start at the left (solid circle), so that you're not climb cutting.
![Font Parallel Circle Diagram Rectangle]()
When you route the slot start at the left (solid circle), so that you're not climb cutting.
