OK, I have a few hours I can work today. I forgot to post these pics last night of the control horn block installation.
I've played with different methods of this and this is the strongest, lightest and easiest method that I have come up with for the CNC cut control horns (ARF style). This is basically semi-lite balsa blocks. I use this because it grabs the foaming glue the best out of all materials that I tried.
The blocks will be glued on four of the six sides so one side with the slots needs to be taped off. Put some tape and trim off so the slots are covered and most of the wood surface is showing. This keeps the foaming glue from filling up the slot.
First put tape on the cut-out control surface and trace the block where you want it. Typically I just trace it with my hobby knife so I get an exact fit. Then take several passes to carefully score through the sheeting laser. The depth of the exacto blade is perfect for the block depth, so cut around it carefully and make sure that the back cut is perpendicular to the surface of the balsa. If done correctly the block should fit nicely in the slot, the top should stick up just a hair and the front should angle out slightly as shown.
Install these with Gorilla quick glue. Use a spreader and spread a thin layer of glue on all the inside walls of the hole you just cut. Mist the block with water (don't soak, just mist) and insert it, wipe off any excess glue that comes out. Now securely tape it in place pushing the tape around the block to keep the glue from foaming out.....you want it to foam into the foam core. You also don't want it to push the block out.