RE: Experienced with the TWM P-51 GS?
The top of the tank compartment was 1/8" or 3.5mm lite plywood. Barry has concerns about the OS GT33 ignition module fitting on top of it, but I don't think there is an issue as these pictures show.
The original piece of plywood weighed 0.8 ounces. I made a new 1/8" aircraft plywood piece and it weighed 1.2 ounces before I cut a hatch into it. I will use the hatch to get inside the tank compartment area if needed. I will use light plywood as the hatch and the weight gain will be minimal. I can get my whole hand into the hole. There is enough material at the front to add strength to the firewall and I will use chopped carbon fiber like I did around the bottom, plus I will pin the plywood to the firewall in 2 to 4 spots. The angle will be made of 1/8" aircraft plywood as well. So the nose should be sturdy.
With all three of my tail servos in the rear, I have a little more ability to add weight in the front than I would have had otherwise. But I'm still trying to keep it down. And since I don't have two retract servos up front that is another issue for weight up front. With the tuned pipe instead of a muffler, that is also a factor in allowing me to reinforce as I see fit.
I will complete the velocity stack cover, then do some more work on this area, and hopefully get the engine compartment wrapped up in a week so I can cover the fuselage.