Since the plan is to "tweak" the airframe a bit to carry additional weight I wasn't ready yet to glue it together as per assembly instructions and decided instead to provisionally wrap it in plastic wrap for sandwiches, to hold it together for a short test flight.
Before that, was fully equip with all the heaviest components:
- battery pack
- servos & receiver (located on the outside)
- motor controller
- three packs of £ coins*
With about 760g it was ready for a series of short flights (no RC control, the battery has yet to be charged).
The most "successful" was the second arrangement with the battery at the back of carved out space and the simulated G1 in the fuselage, closes to the beginning of the wings.
In fact it was a bit too successful, since it kept flying after the desired couple of meters.
P.S. Please excuse my brothers laughter at the end, he only meant well.