Here we just cut about 10mm down to the outline of the cored part of the landing brake. This is where the brake fits into the fuselage and then the glass to glass area is outside of this outline by 0.9" or whatever you like to make up the final size of the brake.
I used a router with a 45 degree bevel around the inside of the outline. Then I made a box for the router so I could cut to a depth easily. Should be a handy tool (someone elses idea).
The 3rd pic you can see I've just cut an insert for what will be the core of the brake. There was a bit of fiddling to get this done and most of that depth will be lowered a little later. I just tried to start out neatly.
