Here's the shim. I had to remove the neck to rout this bridge cavity.
This line is where I need to rout to.
So I set up a fence 2 1/4" away and routed a groove deep enough to get the collar bushing to keep the edge, then finished the rout using that edge as a guide. I had to flip it over and finish the back by hand with a dremel.
The new cavity
From the back
And then I reassembled it.