I would add a double panel of balsa, let into the ribs just behind the spar, 10" long by 2" wide. Cut a slot 8" long x 1" wide for the blade and let this into the ribs so that the profile is maintained. this should ideally be outboard of the tailplane. The bottom sheet can be 1/16th", but the top sheet should be 1/8". The blade should be made from 1/16" carbon sheet faced with 1/16" balsa, hinged at the front by small flat kavan style hinges.. Now you need to actuate the spoilers. Two ways: 1. Put a small servo under each blade to actuate them. The downside is that you need to make a hatch underneath to get to the servo for set up and matenance. 2. use a length of plastic covered steel fishing trace to pull each spoiler up and a very light return spring. Run the fishing trace through some snake tube curved around in a long curve and exiting the underside of the wing root facing forwards. Move your rudder servo to the space under the wing and have a spoiler servo in its place. Make loops in the end of each length of fishing trace about 1/2" long or a little less. Screw a small cheese head screw into your servo arm and pop the loop onto it. You need to get the length just right so do all this prior to covering. The spoilers need to move together and close completely flat. Cover the spoiler and fix in place prior to covering the wing to ensure its still works when covered.
This model does not need flaps or airbrakes. It already flies slowly and has oodles of wing area. Its not an F86 and airbrakes are not needed, and indeed they work on the principle of the drag increasing at the square of the speed. It does not have much speed to begin with. Spoilers work proportionally and are ideal on slow models. Mine had been flying 40 years. Its does not have spoilers so I use the rudder.
John