If you take that door off on the right, can you cut a piece of wood the depth of your armoir and clamp it to that side in the front and back?
If so, you can clamp that wood horizontal and then just run the router along it and take off 1/8" depth and then keep moving that piece of wood up as you cut out a strip. Larger router so you can do a wide swath at a time, not a trim router. Well, aren't you retired? Trim router would work too...
🙂
The better way is if you can clamp two pieces apart from each other that you can move the router up/down without it sliding off one of those two rails so you can cut a very wide swath.
I did something similar recently to route out the bottom of a slab of wood for a mantle so it would sit over the uneven bricks.
Description horrible, but something like this picture:
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