I scratched my head over this so you don’t have to. Making
it doesn’t take much time.
Take a short length of 2×4 and using the router base or a
compass draw the shape on the 2×4. Then band saw out
the half-circle. I had to make a notch for the rack (as in
rack and pinion) on the side of my router.
I ripped the 2×4 piece down to about 2.75” wide to get
rid of roughness on the edges and the rounded corners.
Since the router is 3.5” diameter you don’t need much
more than half that (1.75”) to cradle half the router.
Then resaw the piece of 2×4. I used the table saw to
to the resawing because I used a 1/4” bandsaw blade
to saw out the circle and didn’t want to fuss with
switching to a ripping blade. Now you have two nearly
identical mounts and if your bandsaw blade is running
square (front to back and side to side) they will be
Space the mounts out and measure the gap. Mill up
some blocks to fill the gap. To glue it up you see
how I used wax paper on a piece of MDF? The MDF
is flat, the wax paper prevents the assembly getting
glued to the MDF and I don’t have mess around with
dowels or any other alignment devices.