Wow, we did not know that it would have to be this light! Our intentions were to build a very light fuselage but then try and incorporate 3D printing, so that we can make scale replicas of the wings that our company actually makes. Then we was hoping to have different profiles to show the differences in lift and drag etc. but obviously this option would be far too heavy given what you said.
We were going to have a single motor (or maybe 2 EDF, 1 providing power and 1 dummy for aesthetics) and a single servo motor to control the elevators. We was also going to run the power cables for these motors down the tether to allow us to use a constant power supply and a control board, removing the need for a PCB and transmitter.
One idea that was suggested was to produce a hollow fuselage and place a helium filled balloon inside. Or maybe just source a larger cage (5m is our max unfortunately) what would be your take on this? given that we would be probably looking at an easy 300g for the model.