First and foremost is the set of carbide router bits I picture below. It took me a few tries to zero in on this process but the smallest of the shown router bits did 95% of the slotting/cutting. And I found out that marking (delineating?) the spots to cut is best accomplished with masking tape as you'll see.

The littlest router bid is indispensable.

Slot the flap and use slot to locate control opening.

Just about exactly one inch

I notched out each flap horn to clear axle rods. I still had to section out the axle tube in the flaps to clear.

Here's a general idea of the size of the flap servo rod opening. Smaller than I expected! I started the hole thru the flap side to locate then ream most from the servo pocket.

"marking" the flap for the slot. I used this process for ALL cuts.